Sen. Elizabeth Warren assailed by rivals in 4th Democratic debate

The Democratic presidential contenders were in Westerville, Ohio, for the fourth primary matchup of the season.

The pivotal debate, held on the campus of Otterbein University, came as the Democratic field is sparring -- over health care, immigration, climate change, criminal justice reform, among other topics -- and as an ongoing impeachment inquiry battle pulls the contest into Washington’s orbit despite efforts to keep kitchen table issues at the forefront.

Here is how the night unfolded.

11:44 p.m.: Warren takes fresh heat in scattered Democratic debate: ANALYSIS

The stage was set with a new set of issues, a new candidate, and -- arguably -- a new frontrunner.

That new front-runner took the harshest attacks Tuesday night. Yet Sen. Elizabeth Warren was targeted mostly over old disagreements -- with fireworks that illuminated familiar ground, or that created what Sen. Cory Booker called “déjà vu all over again.” Read Rick Klein's analysis here.

11:25 p.m.: Trump campaign reacts to Dem debate

The Trump campaign is out with a statement after tonight's debate, declaring again that "President Trump was the hands-down winner."

"A dozen Democrats stood on stage tonight and made several things clear," Trump campaign manager Brad Parscale said. "They have always wanted to impeach President Trump, they have always wanted to eliminate employer-provided health insurance, they have always wanted to raise your taxes, and they have always wanted to unravel the hottest economy in modern history. Another thing was also clear: Once again, President Trump was the hands-down winner of this debate."

ABC News' Will Steakin contributed to this report.

11:19 p.m.: Several members of "The Squad" endorse Sanders

Two senior Sanders campaign sources confirm that progressive freshman Rep. Alexandria Ocasio Cortez plans to endorse veteran progressive stalwart Sen. Bernie Sanders and will be “special guest” at his rally in New York this weekend.

Similarly, Rep. Ilhan Omar's office announced that she, too, is endorsing Sanders’ bid for the White House.

“Bernie is leading a working-class movement to defeat Donald Trump that transcends generation, ethnicity, and geography," Omar said in a statement.

Sanders, in turn, wrote "Ilhan is a leader of strength and courage. She will not back down from a fight with billionaires and the world’s most powerful corporations to transform our country so it works for all of us. I’m proud of what we’ve done in Congress, and together we will build a multiracial working-class coalition to win the White House.”

ABC News' MaryAlice Parks and Adam Kelsey contributed to this report.

11:11 p.m.: 5 Key takeaways from the 4th Democratic debate

Democrats were (mostly) united on impeachment. Warren faced the scrutiny of a front-runner, Biden got support amid attacks. Democrats rumble on health care again. Mayor Pete came out swinging. Sanders rebounded after his heart attack.

Here are five key takeaways from Tuesday night's fourth Democratic debate.

11:07 p.m.: The debate has ended

The fourth Democratic debate of the primary season, featuring 12 candidates, is now over.

The final question, on surprising friendships, drew surprising responses on social media--including from some of the candidates.

"Three hours and no questions tonight about climate, housing, or immigration. Climate change is an existential threat. America has a housing crisis. Children are still in cages at our border. But you know, Ellen," former HUD Secretary Julian Castro wrote on Twitter.

11:06 p.m.: Top-tier weighs in on surprising friendships

Sen. Bernie Sanders: "I worked with a very conservative Republican from Utah Mike Lee. And he understood, we disagree on everything, that the U.S. Involvement in the Saudi led war in Yemen was a catastrophic disaster for the people of Yemen."

Sen. Elizabeth Warren: "You asked about a surprising friend. For me, it would be Charles Freed. 27 years ago, when I was under consideration for a job, he was someone who had been, George Bush the first, solicitor general. A deeply principled Republican. And we didn't agree on much. I was far more liberal than he was, but he also was willing to listen to my work about what's happening to America's middle class. And Charles engaged with it over and over and ultimately is the person who made sure I got the job."

Former Vice President Joe Biden: "John McCain, he worked for me when he worked in the Navy. And he was my---he was assigned to me to travel around the world. We became close friends. Became very close friends with my wife, Jill."

11:03 p.m.: More Democrats respond on surprising friendships

Former Texas Rep. Beto O'Rourke: "My flight had been snowed in, in Washington D.C. I happened to be in the elevator with a Republican member of Congress, Will Hurd. And on a whim, I said do you want to rent a car and drive to Washington...A Republican and a Democrat finding out what we had in common. By the end of that trip, not only had we formed a friendship, but we had formed trust."

Sen. Cory Booker: "I was the mayor of a large city with a Republican governor. He and I had to form a friendship even though I could write a dissertation on our disagreements.

Tech entrepreneur Andrew Yang: "The friendship that sticks out is Fred. Avid Trump supporter, a trucker. He spent time in jail. I heard about his experiences trying to get people off drugs. And I'm happy to say that, after our ride together, he said he would move from Donald Trump to my campaign which was a thrill for me. And we’ve remained in touch ever since."

California Sen. Kamala Harris: "Probably Rand Paul. He and I -- actually I invited him to join me on a bill to end the money bail system in the United States. He and I agree on almost nothing. But we agree on that."

Mayor Pete Buttigieg: "I think about the friendships that I formed in the military. People who were radically different from me---Different generation, different race. Definitely, different politics. And we learned to trust each other with our lives."

10:51 p.m.: Democrats talk surprising friendships that have influenced them

Castro: "I believe that we should be more kind to other folks. I also believe that we should hold people to account for what they've done, especially public servants who have a record of having done something or not done something."

Gabbard: "We're all God's children. So I've developed friendships that some people may be surprised about within the Washington circles, especially with Republicans like Trey Gowdy, for example. He and I disagree a lot, and very strongly, on a lot of political issues. We've developed a friendship that's based on respect, and he's been there for me during some personally challenging times."

Klobuchar: "For me, it's John McCain, and I miss him every day...There is nothing more liberating in life than fighting for a cause larger than yourself. That's what we're doing right now."Steyer: "So I'm friends with a woman from Denmark, South Carolina, named Deanna Barry, who is fighting for clean water and environmental justice in her community. She's a different gender. She's a different race. She's from a different part of the country, but she reminds me of my parents in terms of her courage and her optimism and her honor."

10:35 p.m.: Joe Biden, sandwiched between the progressives, takes aim at their 'vague campaigns'

When asked about if both Sens. Bernie Sanders, who is calling for a political revolution, and Elizabeth Warren, who is calling for big, structural change, could attract the kinds of voters Democrats need to win back the White House, former Vice President Joe Biden said, "I think their vision is attracting a lot of people, and I think a lot of what they have to say is really important. But, you know, Senator Warren said we can't be running any vague campaigns. We've got to level with people. We've got to level with people and tell them exactly what we're going to do, how we're going to get it done, and if you can get it done."

"I'm the only one on this stage that has gotten anything really big done," he added. "Both are being vague on the issue of -- the -- Medicare for all."

Sanders struck back, telling the vice president, "In two ways. Joe, you talked about working with Republicans and getting things done. But you know what you also got done, and I say this as a good friend. You got the disastrous war in Iraq done. You got a bankruptcy bill which is hurting middle-class families all over this country."

"If we have the guts, and I would like to see the Democratic party have that guts, to stand up to the drug companies and the insurance companies and tell them that the function of health care is to guarantee care to all people, not to make $100 billion in profit," Sanders said.

But Biden asserted, "We can do that without Medicare for All."

Then Warren jumped in, talking up her time serving in the Obama administration, saying, "I had an idea for a consumer agency that would keep giant banks from cheating people."Biden fired back while pointing at Warren, "I agreed with the great job she did, and I went on the floor and got you votes. I got votes for that bill. I convinced people to vote for it, so let's get those things straight too."

Warren then replied, "I am deeply grateful to President Obama, who fought so hard to make sure that agency was passed into law."

10:25 p.m. Mayor Pete Buttigieg and Julian Castro lay out their plans for the Supreme Court

While making clear that he does not believe in packing the court, Buttigieg said he does want to increase the size of the U.S. Supreme Court.

"We cannot go on like this, where every single time there is a vacancy, we have this apocalyptic, ideological firefight over what to do next," Buttigieg said. "Now, one way to fix this would be to have a 15-member court where 5 of the members can only be appointed by unanimous agreement of the other ten."

Castro pushed back on this idea saying that he would prefer implementing term limits on Supreme Court justices.

"You know, I think the plan that Mayor Pete mentioned is an interesting one, but I actually believe if we were selecting from one of those things, that the smarter move might be to look at term limits or having people cycle off from the appellate courts, so that you would have a replenishment of perspective," Castro argued.

10:23 p.m.: Biden says no to 'court-packing', insists on codifying Roe v. Wade

Former Vice President Joe Biden said he would not engage in tactics that use the Supreme Court to achieve ends that would be better served by Congress.

"I would not get into court-packing," Biden said when asked if he would seek to add additional justices to the Supreme Court. "We begin to lose any credibility the court has at all.To ensure the longevity of women's reproductive rights," Biden said. "I would make sure we move and insist that we pass, we codify Roe V. Wade."

10:22 p.m.: Senators tackle reproductive rights as debate turns to the issue for the first time

After multiple calls for a discussion of reproductive rights, the moderators asked a slate of candidates to respond to questions on how they will support reproductive rights.

When asked about states restricting abortion, Sen. Kamala Harris said of her plan to stop them, "My plan is as follows. For any state that passes a law that violates the constitution, and in particular, Roe V. Wade, our Department of Justice will review that law to determine if it is compliant with Roe V. Wade and the constitution and if it is not, that law will not go into effect. That's called pre-clearance."

"But, you know, I'll go further," she continued. "The reality of it is this is still a fundamental issue of justice for women in America. Women have been given the responsibility to perpetuate the human species. Our bodies were created to do that, and it does not give any other person the right to tell a woman what to do with that body. It is her body. It is her right. It is her decision."

Sen. Amy Klobuchar lambasted President Trump for not being on the "side of women."

"I would codify Roe V. Wade and make it the law of the land," she said. "You, Donald Trump, are not on the side of women. You are not on the side of people of this country when over 75% of people want to keep Roe V. Wade on the books when over 90% of people want to make sure we have available contraception. You defunded Planned Parenthood. I would fund it again."

Sen. Cory Booker also weighed in arguing, "They're not just attacks on one of the most sacrosanct ideals in our country, liberty, the ability to control your own body, but they're particularly another example of people trying to punish, trying to penalize, trying to criminalize poverty because this is disproportionately affecting low-income women in this country. I will create the office of reproductive freedom and reproductive rights in the white house and make sure we begin to fight back on a systematic attempt that's gone on for decades to undermine Roe V. Wade," he said.

10:12 p.m.: Kamala Harris calls for shutting down Donald Trump's Twitter account.

Sen. Kamala Harris pivoted to Donald Trump's Twitter habits while answering a question about the effectiveness of breaking up big tech companies, saying that the president's account should be taken down in a response to Sen. Elizabeth Warren.

"I would urge you to join me because here we have Donald Trump, who has 65 million Twitter followers and is using that platform, as the president of the United States, to openly intimidate witnesses, to threaten witnesses, to obstruct justice. And he and his account should be taken down," Harris said.Warren seemed to demur, saying "I don't just want to push Donald Trump off Twitter. I want to push him out of the white house. That's our job."

10:11 p.m.: Tom Steyer makes his pitch in his debut debate performance

Billionaire Tom Steyer, who earned a spot on the debate stage Tuesday night for the first time since announcing his candidacy in July, made his pitch for why he's running for office: "I agree with Senator Warren that, in fact, monopolies have to be dealt with. They either have to be broken up or regulated, and that's part of it."

"We have to understand that Mr. Trump is going to be running on the economy. He's going to be saying he's the person who can make it grow," he said. "I started a business from scratch, one room, no employers, and built a multi-billion dollar international business. We're going to have to show the American people that we don't just know how to tax and have programs to break up companies but also talk about prosperity, talk about investing in the American people, talk about harnessing the innovation and competition of the American private sector."

He added: "In fact, if we want to beat Mr. Trump, I think somebody who can take -- go toe to toe with him and show him to be a fraud and a failure as a business person, and a fraud and a failure as a steward of the American economy, is going to be necessary."

10:05 p.m. Democrats debate over breaking up tech giants

While agreeing with Sen. Elizabeth Warren that there a problem with competition among big tech companies, tech entrepreneur Andrew Yang took the position that breaking up these companies does not go far enough.

"As usual, Senator Warren is 100% right in diagnosing the problem," Yang said. "There are absolutely excesses in technology and, in some cases, having them divest their business is the right move. But we also have to be realistic that competition doesn't solve all of the problems.

He added, "So it's not like breaking up these big tech companies will revive main street businesses around the country. And as the parent of two young children, I'm particularly concerned about screen use and its effect on our children. Studies clearly show that we're seeing record levels of anxiety and depression coincident with smartphone adoption and social media use. Breaking up the tech companies does nothing to make our kids healthier."

Warren disagreed saying she's "not willing to give up and let a handful of monopolists dominate our economy and our democracy."

"We need to enforce our antitrust laws, break up these giant companies that are dominating big tech, big pharma, big oil, all of them," Warren said.

10:01 p.m.: Tulsi Gabbard says argues being commander-in-chief is the key qualifier for president

Amid questions about if age matters when choosing a president, Hawaii Rep. Tulsi Gabbard, 38, said, "I was going to say it's not fair to ask these three about their health and their fitness to serve as president but not every other one of us. I am grateful to have been trained very well by the army and do my best to stay in shape, but here's the real question I believe you should be asking is: who is fit to serve as our commander in chief?"

Trump's decision-making on foreign policy, Gabbard argues, "highlights how critical it is that we have a president and commander in chief who is ready on day one, bringing experience and understanding in foreign policy and national security."

9:57 p.m. Age of 2020 contenders comes under question

After Sen. Bernie Sanders tried to assured voters that he is healthy and up to the task of being president of the United States, former Vice President Joe Biden was asked how certain he is that he can handle the job commander in chief at the age of 80.

"Because I’ve watched it. I know what the job is," Biden said. "I've been engaged. Look, one of the reasons I'm running is because of my age and my experience. With it, comes wisdom. We need someone to take office, this time around, who on day-one can stand on the world stage, command the respect of world leaders from Putin to our allies, and know exactly what has to be done to get this country back on track."

Sen. Elizabeth Warren was also asked to defend why at her age she believes she can beat President Donald Trump versus a younger candidate.

"I will outwork, out-organize, and outlast anyone, and that includes Donald Trump, Mike Pence, or whoever the Republicans get stuck with," Warren said. "The way I see this, the way we're going to win is by addressing head-on what millions of Americans know in their bones, and that is that the wealthy and the well connected have captured our democracy, and they're making it work for themselves and leaving everyone else behind."

9:55 p.m.: Candidates discuss whether age is an issue

With three of the Democratic candidates in their 70s, will age be a factor in determining the nominee?

9:55 p.m.: Bernie Sanders addresses health after heart attack

During the debate, which marks his official return to the campaign trail since suffering a heart attack, Sen. Bernie Sanders, 78, said, "I'm healthy. I'm feeling great, but I would like to respond to that question."

Sanders then launched into a response about the opioid crisis, asserting, "I wanted to pick up on what Kamala and Cory and others have said. Let's take a deep breath, take a look at this opioid epidemic. You have executives, CEOs of major pharmaceutical companies, making tens of millions of dollars a year ... This is what unfettered capitalism is doing to this country, and it's not just the drug companies."

He then further addressed concerns about his health, saying, "Let me invite you all to a major rally we're having in queens, New York ... we are going to be mounting a vigorous campaign all over this country. That is how I think I can reassure the American people. But let me take this moment if I might to thank so many people from all over this country, including many of my colleagues up here, for their love, for their prayers, for their well wishes. And I just want to thank you from the bottom of my heart, and I'm so happy to be back here with you this evening."

9:50 p.m. 2020 contenders address how they will tackle the opioid crisis

Sen. Amy Klobucher took on manufacturers during the first question on ending the rural crisis in rural America.

"So my first answer to that question, in which is included in my plan, is that the people who should pay for this, who should pay for the treatment are the very people that got people hooked and killed them in the first place and that is the people that are manufacturing these opioids," Klobuchar said to applause.

Former congressman Beto O'Rourke echoed a former controversial proposal from his time as a city councilman in Texas, that opioids should be decriminalized.

"I remember a veteran telling me that he bought heroin off the street because he was originally prescribed an opioid at the VA," O'Rourke said. "Now imagine if that veteran, instead of being prescribed an opioid, had been prescribed marijuana because we’ve made that legal in America, ensured the VA could prescribe it, expunge the arrest record for those who have been arrested for possession and made sure that he was not prescribed something to which he would become addicted."

"I am not going to give these police officers another reason to go door to door in certain communities, because police violence is also gun violence, and we need to address that," he said.

9:48 p.m.: Elizabeth Warren dominates speaking time at Democratic debate

Massachusetts Sen. Elizabeth Warren has so far dominated Tuesday night's Democratic debate in terms of speaking time, largely because she's fielded harsh criticism from fellow candidates. Warren had talked for about 10:50 through the first hour -- twice as much as any rival. She took far more heat from peers than anyone else, including former Vice President Joe Biden.

9:47 p.m.: Julian Castro invokes Atatiana Jefferson during response on gun violence epidemic

In the middle of his answer on preventing gun violence with handguns, former HUD Secretary Julian Castro invoked the incident with Atatiana Jefferson, a 28-year old woman who was fatally shot over the weekend while in her home by a Fort Worth, Texas cop who was responding to a call for a welfare check: "A cop showed up at 2 in the morning at her house when she was playing video games with her nephew and within four seconds he shot and killed her through her home window. She was in her own home."

9:43 p.m. Joe Biden defends his record on taking on the National Rifle Association

"I'm the only one on this stage who has taken on the NRA and beat them and beat them twice," Vice President Joe Biden said while laying out his proposal on gun safety.

9:46 p.m.: Trump campaign weighs in on Elizabeth Warren's debate tactics

Sen. Elizabeth Warren is facing attacks from nearly all sides at tonight's debate, and the Trump campaign is predictably unimpressed with her performance."Unable to be honest about raising taxes to pay for her healthcare plan. So obvious that she’s dodging and not doing it well," Trump campaign communications director Tim Murtaugh told ABC News.

ABC News' Will Steakin contributed to this report.

9:42 p.m.: Elizabeth Warren reiterates support for eliminating the filibuster

In her answer on gun reform, Sen. Elizabeth Warren reiterated her support for eliminating the filibuster, arguing, "I stood in the United States Senate in 2013 when 54 senators voted in favor of gun legislation and it didn't pass because of the filibuster."

"We have got to attack the corruption and repeal the filibuster or the gun industry will always have a veto over what happens," she demanded.

9:32 p.m.: Beto O'Rourke and Mayor Pete Buttigieg have testy exchange as debate turns to gun safety

Beto O'Rourke was pushed on how he would enforce his staple gun policy, a mandatory gun buyback program.

"If someone does not turn in an A.R.-15 or A.K.-47, one of these weapons or war or brings it out in public and brandishes it in an attempt to intimidate, as we saw when we were at Kent State recently, then that weapon will be taken from them," O'Rourke said. "If they persist, there will be other consequences from law enforcement.But the expectation is that Americans will follow the law."

After weeks of criticism leading up to this face-to-face debate, Mayor Pete Buttigieg spar in person over this gun buyback program.

"Congressman, you just made it clear that you don't know how this is actually going to take weapons off the streets. If you can develop the plan further, I think we can have a debate about it. But we can't wait. People are dying in the streets right now," Buttigieg said as he criticized O'Rourke's vision. "We can't wait for purity tests, we have to just get something done."

O'Rourke fired back in a swipe at the mayor saying, "Listening to my fellow Americans, to those students who March for our lives, who came up with this extraordinary bold peace plan, let follow their inspiration and lead and not be limited by the polls and consultants and focus groups."

"I don't need lessons from you on courage, political or personal. Everyone on this stage is determined to get something done," Buttigieg replied.

9:31 p.m.: Amy Klobuchar tackles election security

As her fellow Democrats discussed the dangerous influence of Vladimir Putin on the world stage, Sen. Amy Klobuchar took on how his influence could affect American elections here at home.

"We have not talked about what we need to do to protect ourselves from Russia invading our election," she said. "This wasn't meddling. That's what I do when I call my daughter on a Saturday night and ask her what she's doing. Sorry. This was much more serious than that. This was actually invading our election."

She then targeted Senate Majority Leader Mitch McConnell for blocking Democratic efforts to safeguard U.S. elections.

"So to protect ourselves in 2020, what we need, one, backup paper ballots in every single state," she asserted. "That is a bill that I need, and we need to stop Mitch Mcconnell from stopping that from happening. And then we need to stop the social media companies from running paid political ads, including ones last time in rubles, without having to say where those ads came from and who paid for them."

9:27 p.m. Yang hammers on the threats on artificial intelligence on democracy

Yang warns that the U.S. is "decades behind" technological advancements and that election meddling is the prime example of this."Russian hacking of our democracy is an illustration of the 20th-century threats," Yang argued. "Artificial intelligence, cybersecurity, climate change, loose nuclear material, military drones and non-state actors. These are the threats that are going to require our administration to catch up in terms of technology."

9:26 p.m.: Democrats' turn up the heat on Trump over checking Russia's Vladimir Putin on world stage

New Jersey Sen. Cory Booker, in navigating a question on how to check Russian President Vladimir Putin, chided President Trump, saying, "This president is turning the moral leadership of this country into a dumpster fire ... We cannot allow the Russians to continue to grow in influence by abandoning the world stage."

"There can be no better friend than the United States of America and no better -- no greater enemy than the United States of America. This president has turned that upside down and now is doing things to undermine our critical alliances and partner with Russia ... This president is making us less safe," he said.

Former Vice President Joe Biden also took on the president when discussing his foreign policy credentials, telling the audience, he is the "only person who spent extensive time alone with Putin as well as with Erdogan," before adding, "We have an erratic, crazy president who knows not a damn thing about foreign policy and operates out of fear for his own reelection."

9:23 p.m.: Booker appears to be on a uniter strategy: ANALYSIS

Sen. Cory Booker came with the most concerted uniter-not-a-divider strategy. First he defended Biden about attacks on his son’s business record against … no one on stage. Then he played the role of worrier about Dem-on-Dem violence: “Tearing each other down because we have a different plan to me is unacceptable.”

ABC News' Rick Klein contributed to this report.

9:23 p.m.: "A very different Mayor Pete Buttigieg": ANALYSIS

We are also seeing a very different Mayor Pete Buttigieg in this debate. Repeatedly he has called out his Democratic colleagues on issues he disagrees with them with a new frankness.

"Well, respectfully, congresswoman, I think that is dead wrong," he said to Congresswoman Tulsi Gabbard after her answer about getting troops out of Syria.Earlier he said Warren was being evasive in her answers about paying for Medicare for all. "A yes or no question that didn't get a yes or no answer."

ABC News' MaryAlice Parks contributed to this report.

9:19 p.m. Sen. Kamala Harris said President Donald Trump has given ISIS fighters a "get out of jail free card"

Harris piled on attacks of Trump's decision to pull U.S. troops out of Syria, thus abandoning Kurdish fighters, adding that he's created a "crisis" and that "dude gotta go.""What Donald Trump has done because of that phone call with Erdogan is basically giving 10,000 ISIS fighters a get out of jail free card," Harris said." And you know who the winner is in this? There are four. Russia, Iran, Assad and ISIS. This is a crisis of Donald Trump's making and it is on a long list of crises of Donald Trump's making and that's why dude got to go. And when I am commander in chief, we will stop this madness."

9:17 p.m.: Veterans Pete Buttigieg and Tulsi Gabbard differ over Syria

After Hawaii Rep. Tulsi Gabbard, a major in the Army National Guard, called for an end to the regime-change war in Syria, Mayor Pete Buttigieg, a Navy intelligence officer, responded, "Respectfully, congresswoman, I think that is dead wrong. The slaughter going on in Syria is not a consequence of American presence, it’s a consequence of a withdrawal and a betrayal by this president of American allies and American values."

"I think we need to get out of Afghanistan, but it's also the case that a small number of specialized, special operations forces and intelligence capabilities were the only thing that stood between that part of Syria and what we're seeing now, which is the beginning of a genocide and the resurgence of ISIS," he continued.

But Gabbard immediately disagreed, arguing, "So really, what you're saying, mayor Pete, is that you would continue to support having U.S. Troops in Syria for an indefinite period of time, to continue this regime-change war that has caused so many refugees to flee Syria, that you would continue to have our country involved in a war that has undermined our national security, you would continue this policy of the U.S. actually providing arms in support to terrorist groups in Syria...because they are the ones who have been the ground force in this regime-change war."

But Buttigieg replied, "What we were doing in Syria was keeping our word ... I would have a hard time, today, looking an Afghan civilian or soldier in the eye after what just happened over there, and it is undermining the honor of our soldiers. You take away the honor of our soldiers, you might as well go after their body armor next."

9:12 p.m.: Rep. Tulsi Gabbard attacks the mainstream media

Rep. Tulsi Gabbard called out debate hosts CNN and the New York Times on claims that they "smeared veterans."

"Donald Trump has the blood of the Kurds on his hand, but so do many of the politicians in our country from both parties who have supported this ongoing regime-change war in Syria that started in 2011, along with many in the mainstream media, who have been championing and cheerleading this regime-change war," Gabbard said.

"Not only that but, New York Times and CNN have also smeared veterans like myself for calling for an end to this regime-change war. Just two days ago, the New York Times put out an article saying that I'm a Russian asset and an Assad apologist and all these different smears. This morning, a CNN commentator said on national television that I'm an asset of Russia."

The congresswoman then asked Sen. Elizabeth Warren to join her calls to for an end to this regime-change war in Syria.

"I think we ought to get out of the Middle East," Warren said. "I don't think we should have troops in the Middle East, but we have to do it the right way, the smart way. What this president has done is that he has sucked up to dictators, he has made impulsive decisions that often his own team doesn't understand."

9:10 p.m.: Debate turns to foreign policy, as Democrats tackle crisis in Syria

Former Vice President Joe Biden took the first question on foreign policy, criticizing President Trump for his decision to withdraw troops from Syria.

"I would not have withdrawn the troops and would not have withdrawn the additional thousand troops who are in Iraq, which are in retreat now, being fired on by Assad's people and the president of the United States saying if those ISIS folks escape from the prisons they're in, they'll only go to Europe and it won't affect us," he said.

He continued his attack on Trump in a follow up, "I would want those thousand troops to be protected by air cover -- those thousand troops having to withdraw under fire -- make it clear they're not going anywhere and have them protected and work my way back toward what in fact needs to be done, protecting those Kurds. They lost their lives. This is shameful! Shameful what this man has done!"

9:00 p.m.: Elizabeth Warren finds herself under siege from Democratic rivals

Entrepreneur Andrew Yang and former Texas Rep. Beto O'Rourke continued the criticisms of Sen. Elizabeth Warren over the wealth tax after recent polling puts her now at the top of the pack.

"Senator Warren is 100% right, that we’re in the midst of the most winner take all economy in history. And a wealth tax makes a lot of sense in principle. The problem is that it's been tried in Germany, France, Denmark, Sweden, and all those countries ended up repealing it, because it had massive implementation problems and did not generate the revenue that they projected," Yang contended.

O'Rourke said, "Sometimes senator Warren is more about being punitive and pitting some part of the country against each other, instead of lifting people up."

But Warren fired back, saying, "I'm really shocked at the notion that anyone thinks I'm being punitive. Look, I don't have a beef with billionaires. My problem is you made a fortune in America, you had a great idea, you got out there and worked for it, good for you, but you built that fortune in America, I guarantee you built it in part using workers all of us help pay to educate."

8:57 p.m.: Sen. Amy Klobuchar gives Sen. Elizabeth Warren a 'reality check' on taxes

In her second attack on Sen. Elizabeth Warren, Sen. Amy Klobuchar pushes back on her colleague's claim that everyone else on stage, other than Sen. Bernie Sanders, wants to protect the wealthy adding that her "idea is not the only idea."

"I want to give a reality check, here, to Elizabeth because no one on this stage wants to protect billionaires," Klobuchar said. "Not even the billionaire wants to protect billionaires.""Understand taxing income is not going to get you income where you need to be the way taxing wealth does," Warren said during her response. "I think as Democrats we are going to succeed when we dream big and fight hard, not when we dream small and quit before we get started."

8:56 p.m.: An hour in, it's clear lower-tier Dems are attacking Warren as a front-runner: ANALYSIS

Nearly an hour in, the difference in front-running status seems clear. Sen. Elizabeth Warren is taking far more heat from her rivals than former Vice President Joe Biden. Nobody wanted to go where President Trump wanted to take them regarding Biden. But there are real fights breaking out over health care and the wealth tax -- core progressive priorities.

ABC News' Rick Klein contributed to this report.

8:53 p.m.: Elizabeth Warren slams entire field on stage over wealth tax, except Bernie Sanders

In a fierce defense of her wealth tax proposal, Sen. Elizabeth Warren took aim at the rest of her rivals sharing the stage over the plan to tackle income inequality, except her fellow progressive Sen. Bernie Sanders.

"My question is not why do Bernie and I support a wealth tax, it's why is it does everyone else on this stage think it is more important to protect billionaires than it is to invest in an entire generation of Americans," she said.

8:49 p.m.: Billionaire Tom Steyer goes after billionaires

Tom Steyer, the lone billionaire on the stage, in his debut debate performance, agreed with Sen. Bernie Sanders on closing the income gap.

"Senator Sanders is right," Steyer began. "There have been 40 years where corporations have bought this government and those 40 years have meant a 40-year attack on the rights of working people and specifically on organized labor ... I would undo every Republican tax cut for rich people and major corporations."

He even urged that's why he's running for president.

"There's something wrong here," he said. "That's that the corporations have bought our government. Our government has failed. That's why I'm running for president."

8:44 p.m. Sen. Bernie Sanders takes on income Inequality

Sanders was asked to explain his comment "that billionaires should not exist."

"The truth is, we cannot afford to continue this level of income and wealth inequality, and we cannot afford a billionaire class whose greed and corruption has been at war with the working families of this country for 45 years," Sanders said. "So if you are asking me, do I think we should demand that the wealthy start paying---the wealthiest, top 1%---start paying their fair share of taxes so we can create a nation and a government that works for all of us, yes, that's exactly what I believe."

8:42 p.m.: Tulsi Gabbard shares support for Andrew Yang's universal basic income

Hawaii Rep. Tulsi Gabbard shared her support for Andrew Yang's universal basic income proposal, saying, "I agree with my friend Andrew Yang. Universal basic income is a good idea to help provide that security so people can make choices that they want to see. This has to do with a bad trade deal that we have seen in the past that have also driven fear towards people losing the way that they provide for their families."

"The value that someone feels in themselves and their own lives is not defined by the job that they have but is intrinsic to who we all are as Americans," she urged. "Whatever we choose to do with our lives. We can't forget that."

Booker echoed Gabbard's sentiment, adding, "We have to put the dignity back in work. Number one, you start having trade deals, not like this thing that the president is trying to push through Congress right now that gives pharmaceutical companies benefits and doesn't put workers at the center of every trade deal."

8:40 p.m. Sen. Elizabeth Warren pivots to trade relations and job loss

Sen. Elizabeth Warren argued to voters that it is bad trade policy, not automation, that should be the main focus of job loss.

"So, the data show that we have had a lot of problems with losing jobs, but the principle reason has been bad trade policy," Warren said. "The principal reason has been a bunch of corporations, giant multinational corporations who have been calling the shots on trade, giant multinational corporations that have no loyalty to America. They have no loyalty to American workers. They have no loyalty to American consumers. They have no loyalty to American communities. They are loyal only to their own bottom line."

8:38 p.m.: Cory Booker backs up Joe Biden about his family and Ukraine

In a staunch defense of the vice president, Sen. Cory Booker admonished the moderators for "literally using Donald Trump's lies."

"The second issue we cover on this stage is elevating a lie and attacking a statesman," Booker said. "That was so offensive. [Biden] should not ourselves and the only person sitting at home that was enjoying that was Donald Trump seeing we are distracting from his malfeasance and selling out of his office."

He then reiterated Sen. Kamala Harris' argument about reproductive rights, pushing for a broader conversation on the issue.

"We're not talking about the clear and existential threat in America. That we are in a state that has had two planned parenthoods closed," he said. "We are seeing all over this country women's reproductive rights under attack. And God bless Kamala. But you know what, women should not be the only ones taking up this cause and this fight."

8:35 p.m. Automation and job loss takes center stage

With automation threatening to wipe out the jobs of tens of millions of citizens, Sanders says he has a plan to ensure jobs are protected.

"Damn right we will, and I’ll tell you why," Sanders said. "If you look at what goes on in America today, we have an infrastructure which is collapsing. We could put 15 million people to work rebuilding our roads, our bridges, our water systems, our wastewater plants, airports, etc."

Yang pushed back on Sanders' plan, arguing that his plan to give people $1,000 a month is better for Americans.

"I am for the spirit of a federal jobs guarantee, but you have to look at how it would actually materialize in practice," Yang said. "What are the jobs? Who manages you? What if you don't like your job? What if you’re not good at your job? The fact is, most Americans do not want to work for the federal government. And saying that that is the vision of the economy of the 21st century, to me is not a vision that most Americans would embrace. Also, Sen.Sanders', description of a federal jobs guarantee does not take into account the work of people like my wife, who is at home with our two boys, one of whom is autistic. If we have a freedom dividend of $1,000 a month, it actually recognizes the work that is happening in our families and communities."

8:33 p.m.: Kamala Harris demands a question on reproductive rights

California Sen. Kamala Harris called out the lack of questions on reproductive rights throughout the current debate, and the previous three.

"Not nearly one word with all of these discussions about health care on women's access to reproductive health care, which is under full-on attack in America today and it's outrageous," she said. "People need to keep their hands off of women's bodies and let women make the decisions about their own lives."

8:29 p.m.: Amy Klobuchar goes on offense against Elizabeth Warren

Amid the first contentious moment of the debate, Sen. Amy Klobuchar weighed in on the debate over health care by taking aim at her colleague, Sen. Elizabeth Warren.

"At least Bernie’s being honest here and saying how he’s gonna pay for this and that taxes are going to go up," Klobuchar said. "I’m sorry Elizabeth, but you’ve not said that, and I think we owe it to the American people to tell them where we’re gonna send the invoice."

"You are making Republican talking points right now in this room by coming out for a plan that's going to do that," she continued about building on Obamacare. "I think there's a better way that is bold that will cover more people. It's the one we should get behind."

Warren defended her stance, responding, "I didn't spend most of my time in Washington. I spent most of my time studying one basic question and that is why hard-working people go broke? And one of the principal reasons for that is the cost of health care."

"The problem we have got right now is the overall cost of health care...I’ve put out nearly 50 plans on how we can fight back and rebuild an America that works," she added."I appreciate Elizabeth's work. The difference between a plan and a pipe dream is something that you can actually get done," Klobuchar hit back.

8:27 p.m. "As someone who wrote the damn bill," Sen. Sanders weighs in on health care

Sen. Bernie Sanders highlights his proposal for Medicare for All and admitted that taxes will increase for all.

"At the end of the day, the overwhelming majority of people will save money on their health care bills. I do think it is appropriate to acknowledge that taxes will go up. They will go up significantly for the wealthy and for virtually everybody, the tax increase will be substantially less," Sanders said.

8:26 p.m.: Pete Buttigieg, Elizabeth Warren spar over 'Medicare for All'

Mayor Pete Buttigieg, who has previously refrained from going after his Democratic rivals, took on Sen. Elizabeth Warren directly while standing shoulder-to-shoulder with the senator."We heard it tonight," Buttigieg said of Warren dodging questions about if middle-class taxes will increase under Medicare for All. "A yes or no question that didn't get a yes or no answer. Look, this is why people here in the Midwest are so frustrated with Washington in general, and Capitol Hill."

"No plan has been laid out to explain how a multi-trillion-dollar hole in this Medicare for All plan that senator Warren is putting forward," he said.

In response, Warren responded about her position, saying, "Whenever someone hears the term Medicare for all who want it, understand what that really means. It's medicare for all who can afford it. That's the problem we have got. Medicare for all is the gold standard. It's the way we get health care coverage for every single American."

Buttigieg rebutted, "I don't understand why you believe the only way to deliver affordable coverage is to obliterate private plans. We could achieve that same big, bold goal ... we're competing to be president for the day after Trump."

"Our country will be polarized, more than now, after everything we have been through, after everything we are about to go through, this country will be more divided. Why divide this country over health care when there's a better way to deliver coverage for all," he questioned.

8:22 p.m. Sen. Elizabeth Warren dodges question on her Medicare for All Plan

Fielding the first question on the economy, Warren failed to answer whether she would raise taxes on the middle class to pay for her Medicare for All plan.

"So, the way I see this, it is about what kinds of costs middle-class families are going to face," Warren said. "So let me be clear on this. Costs will go up for the wealthy. They will go up for big corporations. And for middle-class families, they will go down. I will not sign a bill into law that does not lower costs for middle-class families."

8:20 p.m.: Joe Biden tackles his son's role in foreign businesses

Former Vice President Joe Biden, who has made clear that if elected president, his family would not be involved with foreign businesses, was questioned about why he allowed his son to serve on the board of a Ukraine natural gas company when he was vice president.

"Look, my son did nothing wrong. I did nothing wrong. I carried out the policy of the United States government in rooting out corruption in - in Ukraine," Biden said, before laying into Trump. "[Trump] knows if I get the nomination, I will beat him like a drum."

During a follow-up answer, Biden added, "My son's statement speaks for itself."

"I did my job," he continued. "I never discussed a single thing with my son about anything having to do with Ukraine. No one has indicated I have. We’ve always kept everything separate, even when my son was the attorney general of the state of Delaware. We never discussed anything. So there would be no potential conflict. My son made a judgment. I'm proud of the judgment he made. I'm proud of what he had to say."

8:18 p.m. Former congressman Beto O'Rourke weighs in on President Donald Trump impeachment inquiry

"If you do not hold him to account, if there is not justice, not only have we have failed this moment, our Constitution, and our country," O'Rourke said while saying why he believes impeachment is necessary. "We have failed everyone who has sacrificed and laid their lives down on the line. And we cannot do that."

8:17 p.m. Tom Steyer continues his call for impeachment of President Donald Trump

Steyer first called from Trump's impeachment two years ago, and echoed his sentiments during the debate to voters.

" Two years ago, I started the Need to Impeach movement because I knew there was something desperately wrong at 1600 Pennsylvania avenue, that we did have the most corrupt president in the country and that only the voice and the will of the American people would drag Washington to see it as a matter of right and wrong, not of political expediency," Steyer said. "So, in fact, impeaching and removing the president is something that the American people are demanding."

8:16 p.m.: Andrew Yang says he supports impeachment, but urges for 'new vision'

While Andrew Yang noted he supports impeachment from the debate stage, he urged his fellow Democrats to stop talking about the president.

"I support impeachment, but we shouldn't have any illusions that impeaching Donald Trump will be one, be successful, or, two, erase the problems that got him elected in 2016," he said, before adding, "These are the problems that got Donald Trump elected. The fourth industrial revolution. And that is going to accelerate and grow more serious regardless of who is in the oval office. The fact is, Donald Trump when we are talking about him, we are losing. We need to present a new vision and that includes talking about impeaching Donald Trump."

8:15 p.m.: Tulsi Gabbard reiterates her support for House impeachment inquiry

Hawaii Rep. Tulsi Gabbard, who was one of the last Democratic contenders running for president to support impeachment, said of impeachment, "Trump won his election, and as unhappy as that may make us as Democrats, he won that election in 2016. The serious issues that have been raised around this phone call he had with the president of Ukraine and many other things that transpired around that are what caused me to support the inquiry in the House."

"I think that it should continue to play its course out, to gather all the information, provide that to the American people, recognizing that that is the only way forward."

8:14 p.m. Mayor Pete Buttigieg says Republicans are making a mistake on impeachment

When asked if Democratic lawmakers are making a mistake moving forward with an impeachment inquiry without support from Republican lawmakers, Buttigieg says it's Republicans who are making a mistake.

"Well, it's a mistake on the part of Republicans who enable the president whose actions are as offensive to their own supposed values as they are to the values that we all share," Buttigieg said. "Look, the president has left congress with no choice, and this is not just about holding the president accountable for not just things emerging in these investigations, but actions that he has confessed to on television."

8:12 p.m.: Julian Castro dismisses concerns that impeachment is a distraction

As the impeachment inquiry, a pitched battle on both sides, takes up the first minutes of the debate, away from the focus on kitchen-table issues, former HUD Secretary Julian Castro dismisses concerns that it is a distraction.

"We can walk and chew gum at the same time," he told the crowd inside the debate hall. "All of us are out there every single day talking about what we're going to do to make sure that people cross the graduation stage, that more families have great health care, that more folks are put to work in places like Ohio, where Donald Trump has broken his promises, because Ohio, Michigan and Pennsylvania actually in the latest jobs data have lost jobs, not gained them."

"We have to impeach this president and the majority of Americans not only support impeachment, they support removal," he continued.

8:10 p.m. Amy Klobuchar weighs in on President Trump pulling U.S. troops out of Syria

President Trump has received backlash from both sides on the aisle for his recent decision to remove troops from Syria. Many critics, including Klobuchar, said that we have abandoned longtime U.S. allies.

"I would like to hear from him about how leaving the Kurds for slaughter, our allies for slaughter, where Russia then steps in to protect them, how that makes America great again. And I would like to hear from him about how coddling up to Vladimir Putin makes America great again," Klobuchar said.

8:09 p.m.: Cory Booker weighs in on potential impeachment trial

Sen. Cory Booker, one of the members of the Senate Judiciary Committee, was asked Tuesday night in his first question about if he could be fair in an impeachment trial, calling on all Democrats to be fair.

"We must be fair," he said. "This has got to be about patriotism and not partisanship ... That's what this nation needs in what is a moral moment and not a political one. I swore an oath to do my job as a senator, do my duty. This president has violated his. I will do mine."

8:08 p.m. Sen. Kamala Harris says impeachment process won't take long

With many calling on the Democratic lawmakers to lay out a timetable for their impeachment inquiry into President Donald Trump, Harris says that Trump is the "most corrupt and unpatriotic president we have ever had" and that the impeachment inquiry won't take long.

"But the reality of it is that I don't really think this impeachment process is going to take very long, because as a former prosecutor, I know a confession when I see it," Harris said.

"And he did it in plain sight. He has given us the evidence and he tried to cover it up, putting it in the special server. And there's been a clear consciousness of guilt. This will not take very long. Donald Trump needs to be held accountable."

8:06 p.m.: Joe Biden slams Trump as he reiterates call for impeachment

Former Vice President Joe Biden, when asked about how Democrats are navigating an impeachment inquiry, said, "Trump continued to stonewall what the Congress is entitled to know about his background, what he did, all the accusations in the Mueller report."

Biden continued his hammering of Trump, taking a swipe at the incumbent president, saying "I agree with Bernie, Sen. Sanders -- this is the most corrupt president in modern history and, I think, all of our history."

8:05 p.m. Sen. Bernie Sanders comments on Ukraine, Hunter Biden controversy

With Democrats calling on President Donald Trump's impeachment in the wake of his controversial July call with the Ukraine president, Sanders called that interaction "beyond comprehension."

"I think in terms of the recent Ukrainian incident, the idea that we have a president of the United States who is prepared to hold back national security money to one of our allies in order to get dirt on a presidential candidate is beyond comprehension," Sanders said.

8:03 p.m.: Elizabeth Warren fields first question on impeachment

The brewing impeachment fight consuming Capitol Hill made its way onto the debate stage Tuesday night as the first question."Sometimes there are issues that are bigger than politics," said Massachusetts Sen. Elizabeth Warren when asked why shouldn't voters decide President Trump's fate next year at the ballot box. "This is about Donald Trump. But understand, it's about the next president and the next president and the next president and the future of this country. The impeachment must go forward."

"Impeachment is the way we establish this man will not be allowed to break the law over and over again," she said.

7:54 p.m.: The candidates are now taking the stage

The candidates are now taking the debate stage to applause.

7:46 p.m.: FiveThirtyEight's analysis live blog is live

Tonight is the largest presidential primary debate yet (and reportedly ever) with 12 candidates on the stage. And this is actually billionaire Tom Steyer’s first debate of the primary. He’s also one of the eight who have already qualified for the November debate, so this won’t be the last you see of him — and it may not be the last you see of debates where more than 10 candidates qualify. Read more of FiveThirtyEight's analysis here.

7:13 p.m: Ahead of debate, Pelosi, Schiff defend impeachment probe, lack of formal floor vote

Ahead of the debate, House Speaker Nancy Pelosi and Intelligence Committee Chairman Adam Schiff on Tuesday expressed confidence in Democrats’ ongoing impeachment inquiry and efforts to obtain records and testimony from the Trump administration in court - while defending their inquiry from Republican criticism.

Pelosi defended their timeline without providing any new updates and dismissed questions about Republican calls for a formal floor vote.

“I’m not concerned about anything,” she said. Republicans “can’t defend the president so they’re going to process.”

ABC News' Ben Siegel contributed to this report.

6:30 p.m.: O’Rourke wants to flex some policy muscles beyond guns at Tuesday's debate

Former Texas Rep. Beto O'Rourke's aides walked away from the last debate feeling like he had a breakout moment with his "hell yes we're gonna take your AR-15, your AK-47" line, but didn't get the momentum they were looking for in the wake, multiple sources inside the campaign have told ABC News since that night.

Today, one source told ABC News that the campaign understands one breakout moment, or one jab, is evidently not enough to move the polls for O'Rourke.

But that doesn't mean the strategy -- for the candidate struggling to score more than 2% in polls -- is shifting, that source, briefed on tonight's strategy, told ABC News.

O'Rourke's goal for this debate is "substance over flash." The campaign wants viewers to see O'Rourke as a leader on policies beyond guns, acknowledging that some may view him as a single-issue candidate.

One thing we can count on is that O'Rourke will sell his history of being a political underdog. "Not paying attention to the polls and conventional political wisdom has paid off for him in the past," the source said, pointing to his first congressional race against an incumbent Democratic and, of course, his tight contest for U.S. Senate against Ted Cruz in 2018 while supporting impeachment, universal healthcare, and an assault weapons ban.

ABC News' Lissette Rodriguez and Jeff Cook report contributed to this report.

6:15 p.m.: Warren's campaign details her debate day prep

After flying into Ohio Monday, Warren's campaign says the polling frontrunner did some debate prep with her team.

Earlier on Tuesday, after going on a walk as part of her debate day routine, Warren met with the winner of a contest her campaign held for a supporter to fly out to the debate and attend as one of her guests.

Warren is expected to be one of her rivals top targets during the debate, given her rise in recent polling and a strong showing in fundraising over the last three months.

ABC News' Cheyenne Haslett in Westerville, Ohio contributed to this report.

6:14 p.m.: Bloomberg pens op-ed calling out current Dem field, sparks 2020 speculation

Former New York Mayor Michael Bloomberg penned an op-ed in The Washington Post ahead of Tuesday night's Dem Debate—sparking further speculation regarding a possible 2020 presidential run, despite ruling one out earlier in the year.

Bloomberg, once a Republican, Independent and now a Democrat, criticizes the current 2020 Democratic field in the op-ed, writing: "The country elects a commander in chief, and yet based on the campaign so far, one might think we are electing a legislator in chief — or a prime minister whose party controls a parliament."

ABC News' Will Steakin contributed to this report.

6:09 p.m.: Biden campaign responds to Hunter Biden's ABC interview

Biden Deputy Campaign Manager Kate Bedingfield was just on MSNBC and was asked why they feel it was the right strategy for Hunter Biden to be answering questions about allegations that they have maintained are false and have no credibility.

"This is Donald Trump's game. Look, Hunter Biden has been attacked viciously and personally by Donald Trump for the past three weeks. He sat down this morning and answered every question that was thrown at him, which frankly is a lot more than you can say for Donald Trump and Rudy Giuliani and their ilk who are stonewalling and refusing to answer questions in their own impeachment inquiry, by the way. But, you know, you had Hunter sit down, he answered questions. He's shown he's not going to be bullied by the President of the United States and neither is our campaign," Bedingfield told MSNBC's Chuck Todd

ABC News' Johnny Verhovek contributed to this report.

5:15 p.m.: Here's who is on the debate stage

The debate features a dozen candidates packed on the stage, the largest roster yet to appear in a single primary debate ever.

While the same 10 candidates who participated in the third presidential debate a month ago in Houston, hosted by ABC News and Univision, will appear on stage, both Hawaii Rep. Tulsi Gabbard, who reclaimed a podium after missing the cut last month, and Tom Steyer, a billionaire activist who entered the race in July and will be a newcomer to the stage this time around, will join their Democratic rivals.

The 12 candidates who officially qualified for the debate, in podium order as announced by CNN, include:

The pivotal debate, hosted by CNN and the New York Times on the campus of Otterbein University, comes as the Democratic field is readying to spar over health care, immigration, climate change, criminal justice reform, among other topics, but is being consumed by external forces.

The debate is slated to air at 8 p.m. ET on Oct. 15. The moderators will be CNN anchors Anderson Cooper and Erin Burnett, and New York Times national editor Marc Lacey.

2:13 p.m.: FiveThirtyEight and Ipsos try to answer "Who Will Win The Fourth Democratic Debate?"

If something is going to shake up the race before the Iowa caucuses, it’s likely to be a debate. So ABC News partner FiveThirtyEight teamed up with Ipsos to once again track how Tuesday’s debate, hosted by CNN and The New York Times, affected likely primary voters’ feelings about the candidates. The FiveThirtyEight/Ipsos poll, conducted using Ipsos’ KnowledgePanel, will interview the same group of voters twice, on either side of the debate, to capture both the “before” and “after” picture.

1:47 p.m.: Ahead of the debate, Buttigieg comes out swinging

It's clear Mayor Pete Buttigieg is looking to enter the ring tomorrow night swinging -- now going after 2020 competitors Massachusetts Sen. Elizabeth Warren and Former Texas Rep. Beto O’Rourke... calling Warren out, specifically, for hedging on Medicare for All taxes - dissing small-dollar donations as "pocket change" apropos her recent expansion of her no-big-fundraiser pledge to the general election (despite the DNC's debt.)

ABC News' Sasha Pezenik contributed to this report.

12:09 p.m.: Here's how the candidates are faring on raising money

Candidates have been sending out dire pleas heading into the final days, pushing out a cascade of fundraising emails and running ad blitzes on social media.Despite recent polling that showed his campaign trailing behind some of his 2020 Democratic rivals, the Vermont senator set the tone for third-quarter fundraising Tuesday morning, announcing a whopping $25.3 million haul. The number eclipses Sen. Elizabeth Warren's combined total raised in the first two quarters and leaves Sanders with over $61.5 million in receipts this year. Read more here.

5 a.m.: Here's what to watch for in tonight's debate

For the second consecutive matchup, Former Vice President Joe Biden will stand shoulder-to-shoulder with Massachusetts Sen. Elizabeth Warren, pitting the elder statesman up against the rising liberal stalwart, who is steadily climbing in recent national polling and now shares the top spot with the former vice president.

Biden is walking into the debate hall on less steady ground: between fending off President Donald Trump's unsubstantiated claims about the former vice president's inappropriate behavior surrounding his son Hunter’s Ukrainian business dealings, lackluster fundraising numbers in the third quarter of 2019, and his lead in national polls slipping.

Many of the candidates vying for a spot on the 2020 ticket now have launched thinly veiled jabs against the veteran lawmaker -- maintaining full-throated support, while in the same breath saying they themselves would never allow their cabinet members’ families to sit on a foreign board, as Hunter Biden did during the Obama administration.

Biden pressured the Ukrainian government to oust a prosecutor who ostensibly had been leading an investigation into Burisma, an oil company, and was unpopular in his home country due to a lack of action. However, no evidence has emerged to support Trump’s main allegation that Biden did so to benefit his son, Hunter, who was later added to the company's board of directors. Several international leaders, including senior officials at International Monetary Fund, have criticized the prosecutor and said Biden’s recommendation was justified.

As House Democrats move full-steam ahead with an impeachment inquiry, this will be the first debate in which questions about the matter may be broached.

Meanwhile, Warren -- who has sought to avoid clashing with her Democratic rivals before a national audience so far, instead focusing on her pitch for big, structural change -- might become a key target for the lower-polling candidates who are struggling to make their mark on the electorate.

Flanking Biden on the other side will be Vermont Sen. Bernie Sanders, 78, who after suffering a heart attack, vowed to make it to the debate stage amid questions about his health and ability to keep up with the rigorous pace of the campaign trail.

Despite those concerns, he has already previewed the differences he seeks to make between him and his progressive colleague, Warren.

"There are differences between Elizabeth and myself," Sanders said in an interview with ABC’s "This Week" on Sunday, two days before his first official emergence back on the campaign trail since his heart attack. "Elizabeth, I think, as you know, has said that she is a capitalist through her bones. I'm not."

But beyond the three top-tier candidates, the others senators vying for the White House, including New Jersey Sen. Cory Booker, California Sen. Kamala Harris, and Minnesota Sen. Amy Klobuchar, are likely to take the stage with renewed urgency to turn a breakout moment into a tangible spike, as the crowded field enters the critical four months before first votes are cast in the Iowa caucuses in early February.

On Monday, Booker started to draw dividing lines with his opponents, criticizing the South Bend mayor for equating gun buybacks to a "confiscation" of firearms.

"Calling buyback programs 'confiscation' is doing the NRA's work for them … they don't need our help," Booker tweeted.

During an interview with the Snapchat show "Good Luck America,” Buttigieg said, "I just don't think we should wait to have a fight over confiscation when we can win on background checks and assault weapons ban and red flag laws right now."

Despite avoiding conflicts with other candidates so far in the previous three debates, Buttigieg is coming to the stage with a more aggressive approach.

In the same Snapchat interview, he also hit back at O’Rourke, who has recently took aim at Buttigieg for being a "poll-driven" candidate, telling the platform, "This is a policy disagreement, and it’s about governing. I get it, he needs to pick a fight in order to stay relevant.”

But aggressively targeting another candidate has so far backfired for those who’ve tried, such as Castro, who leaned into his aggressive style at the third Democratic debate, when he questioned Biden’s memory. But even some of his competitors criticized his decision to make an apparent swipe at Biden’s age.

Regardless, the debate will provide another night of contrast that will further crystallize the differences among the Democratic field, that still counts 19, on policy, philosophy and governing - before November's upcoming debate could shrink the stage under more stringent qualifying rules.