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WASHINGTON — Underdog Bernie Sanders came to fight, and front-runner Joe Biden promised a woman vice presidential candidate.

The Democratic debate Sunday began as a genteel conversation over how to address the coronavirus and descended into a testy battle over records and who has the courage to do the right thing. The candidates clashed over issues from bankruptcy policy and same-sex marriage to health care and immigration, highlighting their differing visions for the country.

The debate was hosted by CNN and Univision in the nation's capital with no live audience and 6 feet between the candidates' podiums. There was no handshake as the two men instead elbow-bumped to practice social distancing as the coronavirus ravages the U.S. and shuts down much of American public life.

Here are five key takeaways from the evening.

Biden promises a female running mate

For the first time, Biden committed Sunday to selecting a woman to be his running mate if he's the Democratic nominee, adding that he wants his Cabinet to look like the diverse country.

"I would pick a woman to be my vice president," he said.

Sanders said he was inclined to pick a woman but stopped short of an ironclad promise, saying it's important that his running mate is progressive. "In all likelihood, I will," he said.

Coronavirus clash captures the contrast

The opening 30 minutes, about how to address the coronavirus outbreak, captured the two candidates' messages well.

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Biden said Americans "are looking for results, not a revolution," and need immediate action to mitigate the crisis. Sanders said the virus underscored how fundamentally broken the country's health care and economic systems are, arguing that universal health care and far-reaching policies are needed to mitigate income inequality. Biden pointedly said he'd call on the military to help address the crisis, while Sanders said the use of the National Guard might be appropriate.

Biden followed a familiar playbook — accept Sanders' concerns but resist his prescriptions. He said that his rival was right about the broken health care system but that single payer was not the solution; he said that economic inequality was a real issue but that the imminent crisis required a targeted solution in the form of "a major, major, major bailout package" that rewards individuals, not corporations.

Biden was also eager to defuse the tension as he courts Sanders' supporters, saying at one point, "I don't want to get this into a back and forth in terms of our politics." The former vice president, who is trying to unite the party behind him, let it be known on several occasions when he agreed with Sanders.

Sanders draws blood on Social Security

The most heated part of the debate came when Sanders tore into Biden for a litany of past positions — his stance on the 2005 bankruptcy law (which Biden has agreed to try to unwind), his vote to authorize the Iraq war, his support for the Defense of Marriage Act in the '90s. He also knocked Biden for supporting efforts that would have cut Social Security, such as the Bowles-Simpson commission and balanced budget amendments.

"You're an honest guy. Why don't you just tell them the truth here?" Sanders said in a feisty clash full of interruptions and cries of "Bernie!" and "Joe!"

"We did not cut it," Biden said.

"I know, because people like me helped stop it," Sanders retorted.

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Sanders said Biden lacked the courage to take difficult stances when they were unpopular. Biden said Sanders had previously voted against background checks for gun sales and to grant immunity to firearm manufacturers (two positions the senator from Vermont has since disavowed).

"He's making it hard for me now — I was trying to give him credit for some things," Biden quipped, saying that when it comes to addressing climate change, "we disagree on the detail of how we do it. But we don't disagree on the principle."

Sanders evokes Obama

It was a role reversal: When Biden tore into Sanders for opposing the 2007 immigration bill that would have granted undocumented people a pathway to citizenship, Sanders name-checked President Barack Obama in reference to the so-called Dorgan amendment, which gutted the guest-worker program and ultimately derailed the measure in the Senate.

"Y'know who voted with me on that one, Joe? Barack Obama," Sanders said.

He was correct that Obama, then in the Senate, voted for the amendment, but he neglected to mention what happened next: Obama voted "yes" to advance the legislation to a final vote, while Sanders voted "no" and joined the coalition that killed it.

Team Trump was watching Biden

The Trump campaign kept an eagle eye on Biden during the debate in the hope of amassing an opposition research file on its likely general election opponent. It has paid special attention to his verbal stumbles in recent public events, and it delighted Sunday in pointing out mid-debate that the former vice president, who overcame a speech impediment as a young man, got tongue-tied early on and called the H1N1 virus the "N1H1" virus.

An aide to President Donald Trump also highlighted Biden's suggestion that he'd limit deportations to people who have committed felonies, a sign that the president may try to weaponize the issue in the fall.