It's looking like a two-man race.
The novel coronavirus has thrust the 2020 campaign into unchartered terrain, in which the candidates are campaigning without campaigning, shifting events online, instructing employees to work from home and grinding field operations to a halt.
Sunday night's debate was moved from Phoenix to CNN's studio in Washington, D.C., "out of an abundance of caution," with no live audience, according to the Democratic National Committee.
However disrupted, the race for president continues.
Biden is currently winning the popular vote and leading in the all-important delegate hunt -- after a stunning political revival at the tail end of the first four early nominating contests -- with at least 841 estimated delegates compared with 690 for Sanders.
There are 577 delegates up for grabs on Tuesday across Arizona, Florida, Illinois and Ohio.
Here's how the night unfolded. All times Eastern.
9:58 p.m. Final question asks candidates to address coronavirus concerns
The candidates ended where they started: addressing the novel coronavirus and the concerns of American people in this time of uncertainty.
"Our hearts go out to everyone," Sanders began. "We need to move aggressively to make sure that every person in this country who has the virus, who thinks they have the virus, understands they’ll get all the health care that they need because they are Americans."
"This is a time to move aggressively, dealing with the coronavirus crisis, to deal with the economic fallout," Sanders continued, "but it is also time to rethink America and create a country where we care about each other rather than a nation of greed and corruption, which is what is taking place among the corporate elite."
Biden used his final response to take one last dig at the Trump administration, demanding that there should be national standard for what needs to stay open and what doesn't, pointing to drugstores and large gatherings, respectively.
9:50 p.m. Biden and Sanders spar over electability
Biden, leaning on his lead in delegates, reiterated that he still believes his campaign is best positioned to beat Trump in November.
"The energy and excitement that's taken place so far has been for me," Biden said. "I can go down the list. They are coming out for me, and I didn't even have the money to compete with this man in those states."
"The fact is," he added, "I'm winning overwhelmingly among Democratic constituencies across the board."
The Sanders team, in turn, has harped on his ability to turn out young voters, a key part of his constituency.
9:43 p.m. Biden again admits voting for the Iraq war was a mistake
"I learned I can't take the word of a president when, in fact, they assured me that they would not use force," Biden said. "I admitted 14 years ago it was a mistake to have trusted him. And I'm prepared to compare my foreign policy credentials up against my friend here on any day of the week and every day of the week."
Both candidates also were pressed on their past comments agreeing with authoritarian regimes -- Sanders on his recent comments praising the literacy program in Cuba and Biden on his comments made while vice president and appearing to compliment the same regime.
Sanders didn't walk back from that stance either. He said he condemns authoritarianism "whether it’s in Cuba, whether it's in Saudi Arabia, whether it's in China, or whether it is in Russia" but added it's incorrect to say someone like Castro didn’t have some positive impact on their people.
"The bottom line is," Sanders said, "if we’re going to look at the world the way it is, of course, we're opposed to authoritarianism."
"Look," Biden replied, "the idea of occasionally saying something nice about a country is one thing. The idea of praising a country that is violating human rights around the world is, in fact, makes our allies wonder what's going on."
9:30 p.m. Sanders: Paris Climate Accord 'not a big deal,' continuing fracking 'insane'
"It's not a question of reentering the Paris Accord," Sanders said, saying commitments are not enough. "That's fine. Who cares? It's not a big deal."
"This is not building a few more solar panels or a few more wind turbines," Sanders added. "What this is about is transforming our energy system as quickly as we humanly can away from fossil fuel. It is insane that we continue to have fracking in America."
While Biden said he would not support any new fracking, he did agree with Sanders that the single greatest threat to national security is climate change.
But he pushed back on Sanders, saying the Paris Climate Accord was necessary.
"We're 15% of the problem," Biden said. "We need someone who can deal internationally, who can bring the world together again."
Sanders later backtracked a bit on his comments, saying, "Obviously, the Paris Accord is useful."
9:19 p.m. Biden and Sanders weigh in on immigration
Biden and Sanders agreed on most immigration reforms, with both saying they support sanctuary cities.
The move represented a reversal for Biden, who opposed the policy as a presidential candidate in 2007, previously saying undocumented immigrants arrested by local police should be turned over to immigration officials.
With the candidates largely agreeing, Sanders took it one step further by reminding viewers he's the son of an immigrant. The Vermont senator said he would end I.C.E. raids on the first day of his presidency.
"Kids are scared to death when they come from school, their mom or dad may not be there,” Sanders said. “I will end this on day one, the I.C.E. raids, that have been so harmful to so many people.”
9:10 p.m. Biden commits to choosing a woman as vice president
Biden, for the first time, committed to picking a woman as his running mate.
"There are a number of women who are qualified to be president tomorrow. I would pick a woman to be my vice president," Biden said.
Sanders wouldn't go quite as far in guaranteeing he'd choose a woman, but he did say it would happen "in all likelihood."
"It’s not just nominating a woman. It is making sure that we have a progressive woman," Sanders said. "So my very strong tendency is to move in that direction."
Biden also committed to appointing a black female justice to the Supreme Court, adding, “it's long overdue.”
8:55 p.m. Biden and Sanders weigh in on precautions they're taking as older men in a pandemic
Both candidates, in their late 70s, were asked what they're doing to protect themselves as the novel coronavirus spreads across the country.
Sanders first commented on the way he is campaigning, before turning to more incremental steps, including the pair's pre-debate elbow-bump.
"Last night we had a fireside chat, not a rally," Sanders said. "I love doing rallies, and we bring thousands of people out to the rallies. We're not doing that right now. In fact, our entire staff is working from home. So, on a personal level, what we're doing is I'm not shaking hands. Joe and I did not shake hands."
Biden pointed out that, unlike Sanders' recent heart trouble, he doesn't have any of the underlying conditions to put him at additional risk.
"Well, fortunately, I don't have any of the underlying conditions that you talked about -- that I have to worry about,” Biden said, before explaining the precautions his campaign is taking. "Our staff is all working from home. We are not doing rallies any longer. ... And so I'm taking all the precautions everyone else should be taking.”
8:53 p.m. Sanders: I had the guts to take an unpopular vote
Sanders and Biden sparred over a bankruptcy bill, which Biden voted for while in the Senate, as Biden discussed how he's adopting Sen. Elizabeth Warren's bankruptcy platform as a part of his campaign.
"And I don't have to rethink my position because that's what leadership is about -- having the guts to take an unpopular vote," Sanders said of his vote against the now-unpopular bankruptcy law.
8:45 p.m. Biden takes aim at Bernie's 'revolution,' saying he beat Sanders in Super Tuesday even with less money
"What's a revolution going to do? " Biden asked, questioning Sanders' Medicare for All plan. "Disrupt everything in the meantime?"
Sanders shot back, saying that creating real change and an economy that works for all comes from taking on Wall Street, the drug companies, the insurance companies, and the fossil fuel industry.
Responding, Biden pointed to his campaign policies, saying he doesn't take large contributions.
"I've not accepted a contribution from anybody over $2,800, No. 1," Biden said. "My average contribution is $44."
"Super Tuesday and before that, Bernie outspent me 2, 3, 4, 5, 6 to 1. And I still won. And I didn't have any money. And I still won," said Biden.
Responding, Bernie asked Biden to get rid of his Super PAC.
"Why don't you get rid of the Super PAC, which you have right now, which is running very ugly, negative ads about me," Sanders said.
Biden laughed, saying Sanders had nine Super PACs. Sanders denied this and asked Biden to name them. Biden did not.
8:39 p.m. Biden and Sanders agree: Undocumented immigrants shouldn’t fear to get tested for COVID-19
Asked how to ensure undocumented immigrants feel safe enough to get treatment to help stop the spread of coronavirus, both candidates agreed all persons who feel sick should be able to seek treatment without fear of being deported.
Biden had a slip of the tongue in his answer, referring to an undocumented immigrant as an "alien" before quickly correcting himself.
"Anyone who shows up to be tested for coronavirus or gets coronavirus and is treated would be held harmless,” Biden said. "There are certain things you cannot deport an undocumented alien for -- an undocumented person for -- and that would be one of them.”
Sanders took it as another opportunity to pitch "Medicare-for-All."
"I have been criticized because the proposal for Medicare for All that I introduced includes making sure that undocumented people are also covered," Sanders said. "And right now, we have the absurd situation where undocumented people who try to do the right thing -- they’re sick, they want to go to the doctor, they don't want to spread this disease, are now standing and thinking about when I.C.E. is going to deport them."
8:37 p.m. Sanders 'using a lot of soap'
Sanders said his campaign has canceled rallies, and his team is working from home.
"I love doing rallies and we bring thousands of people out to the rallies. We're not doing that right now," Sanders said. "I'm very careful about the people I am interacting with. I'm using a lot of soap and hand sanitizers to make sure that I do not get the infection."
Biden's team has also canceled rallies and is working from home in an effort to stop the spread of the virus.
"We're doing virtual rallies and virtual town hall meetings," Biden said. "I'm taking all the precautions everyone else should be taking. ... I wash my hands God knows how many times a day. I carry with me, in my bag outside here, hand sanitizer."
8:24 p.m. Biden on economy: 'People are looking for results, not a revolution'
Biden and Sanders sparred over the economic response to COVID-19, with Biden saying that the economic response to the national emergency will not be solved with a change in tax policy.
"We are going to have to have a major, major, major bailout package -- that we do not reward corporations, we reward individuals who in fact are really put to the test here," Biden added.
Sanders turned to economic inequality, arguing that the basis of the economy perpetuates the effects of the national emergency.
"What we have got to do also is understand the fragility of the economy and how unjust and unfair it is that so few have so much and so many have so little," Sanders said.
"People are looking for results, not a revolution," Biden responded. "They want to deal with the results they need right now."
8:20 p.m. Biden says experience battling Ebola would help fight coronavirus
Biden said that a leading voice -- a single voice -- is necessary to deal with the current coronavirus crisis.
Touting experience gained from the Ebola crisis of 2014, Biden said he wouldn't let governors move forward by making their own individual decisions. Instead, if he were president, he said he would provide an overall guide for "all the nation," even saying he would use the capacity of the military to address this issue.
"I would call out the military," he said. "Now, they have the capacity to provide the surge of help that hospitals need and that is needed across the nation. I would make sure that they did exactly what they're prepared to do. They've done it. They did it in the Ebola crisis."
8:18 p.m. Sanders pitches Medicare for All, reimbursement of lost wages amid coronavirus outbreak
Sanders pitched Medicare for All, using the coronavirus outbreak as an example of why, he said, it's necessary.
"Right now, in this emergency, I want every person in this country to understand that when you get sick, you go to the doctor. When you get sick, if you have the virus, that will be paid for. Do not worry about the cost right now, because we’re in the middle of a national emergency," Sanders said.
"If Trump can put -- or the Fed can put -- a trillion and a half into the banking system, we can protect the wages of every worker in America," he added.
8:17 p.m. Biden says responding to COVID-19 has nothing to do with Medicare for All
Biden began drawing contrasts between his and Sanders' plan for health care in the country, attacking Medicare for All and its ability to handle a pandemic.
"And with all due respect to Medicare for All, you have a single payer system in Italy. It doesn’t work there. It has nothing to do with Medicare for All. That would not solve the problem at all," Biden said.
Instead, he drew on the measures needed to respond quickly to a national emergency.
"We can take care of that right now by making sure that no one has to pay for treatment, period, because of the crisis. No one has to pay for whatever drugs are needed, period, because of the crisis. No one has to pay for hospitalization because of the crisis, period," he said. "That is a national emergency, and that’s how it's handled."
8:15 p.m. Sanders doubles down on guaranteeing health care to all Americans
Faced with how he would deal with the national crisis caused by the novel coronavirus, Sanders said that as president he would turn to his Medicare for All plan, ensuring that drug companies would not profit off the pandemic and that all Americans would get the health care they needed, regardless of cost.
"Do not worry about the cost of prescription drugs, do not worry about the cost of health care that you're going to get, because we are a nation -- a civilized democratic society," he said. "Everybody -- rich and poor, middle class -- will get the care they need. The drug companies will not rip us off."
8:13 p.m. Biden: 'The present system cannot handle the surge that is likely to come'
Biden calls up his plan to tackle COVID-19, saying that the country should prepare to build emergency hospitals and provide extra equipment for first responders.
"I agree with Bernie -- we’re in a situation where we have to, now, be providing for the hospitals that are going to be needed -- needed now. The present system cannot handle the surge that is likely to come," Biden said. "So, we should already be sitting and planning where we’re going to put these temporary hospitals."
"We provided these hospitals dealing with these great pandemics, and we were able to do it quickly. And people would have a place to go. But we also have to provide the equipment to protect the first responders, and that’s not being done either," he said.
8:10 p.m. Candidates address how they'd tackle coronavirus
The first question of the night: What would you do as president in the face of coronavirus?
Biden said his "heart goes out to those who have already lost someone or those who are suffering from the virus, and this is bigger than any one of us" before laying out a couple steps.
"First of all, have to take care of those who in fact are exposed or are likely to be exposed to the virus, and that means we have to do testing," Biden said. "Secondly, I would make sure that every state in the union had at least 10 places where they had drive-thru testing arrangements."
Sanders' response included direct criticism of Trump.
"Well, first thing we have got to do, whether or not I'm president," Sanders said, "is to shut this president up right now, because he is undermining the doctors and the scientists who are trying to help the American people. It is unacceptable for him to be blabbering with unfactual information, which is confusing the general public."
8 p.m. 11th Democratic debate begins
The 11th Democratic debate of the presidential primary season kicks off in the nation's capital.
Earlier, Sanders' staff and surrogates held a livestreamed pre-debate show that discussed "policy solutions to some of the greatest challenges facing America today."
These types of livestreams are nothing new to the Sanders campaign, which regularly broadcasts all of the senator's rallies and posts videos featuring conversations and interviews with staffers and surrogates -- especially now amid the COVID-19 outbreak.
ABC News' Adam Kelsey reports.
6:05 p.m. Sanders campaign responds to Biden's campaign on college costs
After Biden's campaign announced on Sunday that the former vice president would adopt a plan offering free tuition to students whose families earn less than $125,000, Sanders' campaign responded by saying such a plan doesn't go far enough.
"It's great that Joe Biden is now supporting a position that was in the Democratic platform four years ago. Now we have to go much further," a statement from Sanders' campaign said. "We need to make all public universities, colleges and trade schools tuition-free for everyone like our high schools are. We need to cancel all student debt. And we can fund it with a small tax on Wall Street speculation."
ABC News' Adam Kelsey reports.
4:30 p.m. Biden campaign announces adoption of tuition-free public college plan
Senior officials from Biden's campaign held a press call Sunday afternoon ahead of the debate, giving clear signals that the candidate is attempting to broaden his coalition and reach out to supporters of Sanders, as a general election matchup with Trump becomes more likely.
Biden's campaign announced that the former vice president would be adopting a plan that would make public colleges and universities free for any students whose families earn less than $125,000 a year -- a version of one of Sanders' plans that he previously proposed with Rep. Pramila Jayapal of Washington state. This comes in addition to Biden's announcement on Friday that he would adopt Sen. Elizabeth Warren's bankruptcy plan, which was first reported by ABC News.
"Vice President Biden," a senior adviser said on the call, "is running for president to rebuild the middle class so that this time, everybody comes along. That means an inclusive coalition. So, Vice President Biden is and as president will continue to be open to the best ideas to make this a reality, frankly, regardless of where they come from."
Biden has also now tweeted about his adoption of both plans from Sanders and Warren, saying he's proud to add the policies to his platform.
The campaign said Biden will attempt to offer an olive branch to Sanders' supporters during tonight's debate, making it clear that there is space for them and their ideas in his campaign.
"We welcome your support," the adviser said, "but we're also going to welcome their ideas, their passion and their commitment to the issues that they care so deeply about."
ABC News' Johnny Verhovek reports.