Democratic presidential hopefuls Joe Biden and Bernie Sanders clashed head-to-head for the first time on Sunday night, appearing in a CNN-Univision debate dominated by the coronavirus but featuring some testy exchanges over their respective records and a commitment to a female vice president.

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The only two viable candidates left in the race to be the Democratic candidate to take on Donald Trump debated for two hours without a live audience, two days ahead of primaries in Arizona, Florida, Illinois and Ohio which could all but guarantee Biden becomes the nominee.

Here are four takeaways:

1. How would you tackle the virus?

Sanders threaded his own policy ideas into his proposals to handle the virus outbreak, while Biden focused almost exclusively on dealing with the immediate public health crisis and economic fallout.

"This is like a war, and in a war you do whatever is needed to be done to take care of your people," the former vice president said, adding that under his proposals "everything that you need in terms of dealing with this crisis would be free."

The 77-year-old repeatedly referred back to his experience dealing with crises in the White House, including the 2014 Ebola outbreak, and stressed the "need to listen to the science again."

He also said he would immediately deploy the military to help with the crisis. "I would call up the military now. They have the capacity to provide this surge help. They have the capacity to build 500-bed hospitals."

Sanders, who like Biden agreed the world needed to work together, argued that the crisis "exposes the weakness and dysfunctionality of our healthcare system," calling again for his Medicare-for-all plan and hitting out at the Trump administration's response to the outbreak.

"First thing we have got to do, whether or not I'm president, is to shut this president up right now, because he's undermining the doctors and the scientists who are trying to help the American people," he said. "It is unacceptable for him to be blabbering with un-factual information, which is confusing the general public."

Biden's pitch as a steady and experienced leader in a difficult time, backed up by detailed plans to deal with the current situation. "We have problems we have to solve now," he said, giving him an edge over Sanders, who tended to revert to his longstanding policy proposals rather than immediate action.

2. Biden commits to female VP

Biden pledged that if he is the nominee his running mate would be female, and also recommitted to putting a black woman on the Supreme Court.

"There are a number of women qualified to be president tomorrow," Biden said. "I would pick a woman to be my vice president."

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Sanders, who looked to be taken off guard by Biden's announcement, said his cabinet would look like America and, pressed by the moderator, added that "in all likelihood" he would pick a woman as running mate.

"For me, it's not just nominating a woman," Sanders said. "It is making sure that we have a progressive woman and there are progressive women out there. So my very strong tendency is to move in that direction."

Potential vice-presidential candidates include Kamala Harris, Elizabeth Warren, Stacy Abrams, Amy Klobuchar and Gretchen Whitmer.

3. 'I agree with Bernie'

As Sanders sought to draw out contrasts with Biden, the former vice president attempted to consolidate support in the party by reaching out to the Vermont senator's supporters.

He endorsed Elizabeth Warren's proposal for bankruptcy reform, saying he had spoken with his former rival two days ago to discuss the issue, but struggled to make a case to young voters. 

The 77-year-old also backed Sanders' plan for free public college tuition for families making less than 125,000 U.S. dollars a year, and said "I agreed with Bernie, someone should have gone to jail" following the 2008 financial crisis.

"We disagree on the detail, but we agree on the principle," Biden argued, adding "this is much bigger than if I'm the nominee or Bernie."

Both men pledged to support and campaign for which ever won the nomination, describing Trump as an "existential threat" and a "pathological liar."

4. 'Go to the YouTube'

Sanders repeatedly questioned Biden over his long voting record on the Senate, leading to some irritable exchanges between the men, though they were less aggressive than in previous debates.

The Vermont senator repeatedly asking Biden whether he had been on the floor of the Senate calling for cuts to social security and funding for veterans and advising viewers to "go to the YouTube" to check if the former vice president was telling the truth.

"All that I would say to the American people, go to the YouTube," Sanders said. "It's all over the place. Joe said it many times. I'm surprised you can defend the change your mind on it, but you can't deny the reality."

Sanders highlighted that while he voted against the Iraq war as well as trade deals and for gay marriage, Biden has been on the other side of the issues in the past. Biden retaliated by pointing out that Sanders voted against the Brady bill, which looked to curb gun violence.

The former vice president also attempted to counter criticisms of Obama era mass deportations, saying no one would be deported in the first 100 days of a Biden presidency and that only felons would be deported subsequently.