Transcript for Democratic debate in Ohio: Dems spar over Biden's son, wealth tax, health care
Reporter: From the get-go tonight in the battle ground state of Ohio, the democratic debate revolved around one issue the candidates all agree on. In my judgment, trump is the most corrupt president in the history of this country. The most corrupt president in modern history and I think all of our history. He is indeed the most corrupt and unpatriotic president we have ever had. Reporter: That is the urgency of impeaching president Donald Every candidate here is more decent, more coherent and patriotic than the criminal in the white house. Reporter: But tonight former vice president Joe Biden also found himself on the defensive. My son did nothing wrong. I did nothing wrong. I carried out the policy of the United States government in rooting out corruption in Reporter: Biden treated president trump's recent attacks on him and his son -- Biden and his son are stone cold crooked. Reporter: -- Almost as a badge of honor. He doesn't want me to be the candidate. He's going after me because he knows if I get the nomination, I will beat him like a drum. Reporter: To be clear, there's no proof to support trump's claim that Biden used his post as vice president to enrich his son or that Biden shaped U.S. Foreign policy to advance his son's interests. By the way, whatever happened to hunter? Where the hell is he? Reporter: But trump continues to attack him relentlessly. Where's hunter? Where is hunter? He's right here. I'm working and living my life. President trump says you're in hiding. Hiding in plain sight, I guess. Not at all. Reporter: Today in an exclusive interview, hunter Biden was also on the defensive. I want to get to the heart of it. Did you and your father ever discuss Ukraine? As I said, the only time was after a news account. It wasn't a discussion in any way. Your dad said "I hope you know what you're doing." I said I do. That was the end of discussion. Reporter: He was chairman of burisma. I was a lawyer for one of the most prestigious law firms in the world. In the list of reasons you gave me for the reason you are on the board, you did not list you were the son of the vice what role did that play. I think it's impossible for me to say I'm on any boards without saying I'm the son of the vice president of the united States. If Joe Biden was not your father, would you have been asked to be on the board of burisma? I don't know. What I regret is not taking into account there would be a Rudy Giuliani and a president of the United States that would be listening to this ridiculous conspiracy idea. Reporter: You never thought this might not look right? You know what, I'm a human. Did I make a mistake? Well, maybe in the grand scheme of things, yeah. But did I make a mistake based upon an ethical lapse? Absolutely not. I never discussed anything about my son that had anything to do with Ukraine. Even when my son was attorney general of the state of Delaware, we never discussed anything so there would be no potential conflict. Reporter: Recent polls show Biden still leading the field but losing ground to Massachusetts senator Elizabeth Warren. There's a new front-runner in town and it is Elizabeth Warren. It was obvious from the start of the debate she's the center of the attention. It wasn't just Joe Biden taking heat. Most of the attention was on Elizabeth Warren. Your signature, senator, is to have a plan for everything except this. Reporter: Her rivals pressed her of hough she would pay for her dramatic overhaul of the health care system. Medicare for all is the gold standard. Reporter: Warren and Bernie Sanders advocate medicare for all, doing away with private health insurance and having government health insurance. The costs will not go up for families and I will not sign a bill into la that raises their costs. Costs are what people care about. I don't think the American people are wrong when they say that what they want is a choice. Reporter: Sanders has made it clear he would raise taxes to pay for this huge new entitlement. Warren has not said how she'd pay for it. I think we owe it to the American people to tell them where we're going to send the invoice. Elizabeth Warren has been asked this question in virtually every debate. She does not want to answer the question about how she would pay for her medicare for all plan. Reporter: She also took heat over her plan to tax ultra millionaires, households with incomes of 50 million or higher to fund on childhood education. Sometimes I think Elizabeth Warren is punitive. Why does everyone else on this stage at this it's more important to protect billionaires than it is to invest in an entire generation of Americans? I want to give a reality check here to Elizabeth because no one on this stage wants to protect billionaires. Not even the billionaire wants to protect billionaires. Your idea is not the only idea. Warren may be getting a boost from Sanders. We are going to be mounting a vigorous campaign all over this that is how I think we can reassure the American people and I'm so happy to be back here with you this evening. Reporter: This weekend Sanders sat down with Jonathan Carl. He insisted at no point has he considered dropping out of this race because of his health. Reporter: You and Elizabeth Warren have pretty close to identical positions on big issues. Not quite. Reporter: What do you say to those who say they would pick her because she's eight years younger, she didn't just have a heart attack and the positions you're pretty much the same. Every American is going to make his or her choice about the candidate they want. Elizabeth Warren has been a friend of mine for some 25 years and I think she is a very, very good senator but there are differences between Elizabeth and myself. Reporter: Tonight Biden and Warren faced health questions based on their age. Both are in the their 70s. And I will release my medical records as I have 21 years of my tax records, as no one else on this stage has done so can you have full transparency as to my health and what I am doing. I will outwork, outorganize and outlast anyone, and that includes Donald Trump, Mike pence or whoever the Republicans get stuck with. Reporter: One of the younger candidates on stage, 38-year-old congresswoman Tulsi gabbard weighed in two. She also an army reserve major. 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 you should be asking, who is fit to self-as commander in chief. Reporter: On Syria, there was agreement that the commander in chief made a huge mistake. We have an erratic, crazy president who knows not a damn thing about foreign policy and operates out of fear for his own reelection. Reporter: In his answer Biden appeared to gesture at Bernie Sanders. What does the president do? He says I believe Vladimir Putin. You're gesturing I'm Vladimir Putin here. No, I'm not. Reporter: And Bernie Sanders said he'll get the endorsement than Alexandria ocasio-cortez. Getting this for Bernie Sanders at this moment is a huge potential boost. Reporter: While there are sharp policy differences and distinct personal differences, these Democrats have plenty of common ground. About a third of the folks on stage are not necessarily going to make the next debate. I think can you start to cross off some names in the foreseeable future. Reporter: The question for democratic voters, who among them is likely to go the distance. I'm David Wright for "Nightline" in New York.
This transcript has been automatically generated and may not be 100% accurate.