WASHINGTON (AP) — Former vice president Joe Biden and Sen. Bernie Sanders launched a new phase of the 2020 presidential election Sunday night, with a one-on-one debate held in a Washington studio without a studio audience.

It reflected the dramatic distillation of a once-sprawling field of Democratic presidential hopefuls down to two white men in their late 70s who represent the party's competing ideological wings. And it underscored this extraordinary moment in American politics and society, as the coronavirus pandemic remakes every aspect of life in the United States.

Here are some key takeaways:

CLEAR DISTINCTIONS. NOT CLEAR THEY CHANGE ANYTHING.

Biden and Sanders spent two hours in a spirited matchup, showing no hesitation about litigating philosophical differences. But there were no moments likely to alter the trajectory of the race.

For any progressive voter who sees 2020 as the opportunity to reshape the Democratic Party and the U.S. government, Sanders made their case. For any Democrat who identifies as a centrist or a more pragmatic liberal or simply a voter desperate to defeat Donald Trump, Biden stuck to his script.

That is precisely what played out during 10 previous Democratic debates. There were just many more candidates on stage taking one approach or the other.

Biden's winning streak since the South Carolina primary on Feb. 29 suggests that his tack has more appeal in the current political environment. And Biden's debate performance – notably more steady than many of his previous efforts on more crowded stages – certainly indicates that he's comfortable with his posture.

Sanders' counter on Sunday, though, makes equally clear that the Vermont senator isn't ready to abandon his second presidential bid, even if it's mostly about pulling Biden and the Democratic Party as far as he can toward his progressive policy vision.

UNITY AGAINST TRUMP GOES ONLY SO FAR

Biden and Sanders were united in their umbrage about how President Donald Trump has handled the coronavirus outbreak. But that didn't keep them from turning the situation into a proxy argument over their competing visions for the role of government.

Minutes into the debate, Sanders said the COVID-19 proliferation is evidence that the United States needs his proposed "Medicare for All" single-payer health insurance system. "We're spending so much money and yet we're not even prepared for this pandemic," he said, bemoaning the "dysfunctionality" of the existing U.S. health care system.

Biden, who advocates adding a "public option" to the existing private insurance system rather than scrapping private insurance altogether, pushed back, noting that Italy already has a single-payer system and is watching its coronavirus mortality rate spike anyway. "We're at war with a crisis. This has nothing to do with co-pays," Biden said.

Taken together, it was a distillation of the competing ideologies that define the Biden vs. Sanders matchup. Biden has moved left over his career, but he still hails from the establishment, capitalist wing of a Democratic Party. Sanders, a "democratic socialist," is continuing to push a fundamental overhaul of the nation's economic and political identity.

WOMAN RUNNING MATE

Biden made news when he said unequivocally that he'd pick a woman as a running mate if he wins the Democratic nomination -- more fully committing to something he's previously indicated he'd do.

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

Sanders didn't go quite as far but said "in all likelihood" he, too would choose a woman. The senator said it wasn't just about choosing a woman as a running mate but making sure she was strongly progressive.

Biden also worked to shore up progressive support by endorsing Sen. Elizabeth Warren's view on bankruptcy protection and met Sanders halfway in backing free college for families making less than $125,000 a year.

It worked with some. "I'm ready for @joebiden to move into the White House and I'm ready for a woman vice president," Rep. Jennifer Wexton, D-Va., tweeted.

LAST CALL FOR MEDICARE FOR ALL

With Biden grabbing a large lead in delegates and looking to further pad it when Illinois, Ohio, Florida and Arizona vote on Tuesday, Sanders' days in the race could be numbered -- but he's taking a last stand behind his signature issue.

Sanders said the nation's current health care system, where most people have private insurance provided by their employers, is expensive and ineffective, letting tens of thousands of people die every year from preventable diseases. He said coronavirus only made things worse.

Biden shot back that responding to the crisis had nothing to do with a "single-payer system." That prompted Sanders to respond that testing and treatment for coronavirus should be free but that all other health care coverage should too, saying "we are a civilized democratic society" that ought to follow the lead of Canada and many other countries.

Medicare for All dominated debate in the Democratic presidential primary for months, but Sanders is the last candidate left advocating it. Sunday could be some of the issue's final moments in the policy spotlight.

BIDEN'S TIGHTROPE: PRACTICALTY VS. PROGRESSIVE OUTREACH

Biden would like to be known as the real "progressive" in the race. But he has a different definition of progress than Bernie Sanders and his supporters – a gulf that could become a key factor in the November election if Biden is the nominee.

"We have problems we have to solve now," Biden said during Sunday's debate. "We want a revolution? Let's act now."

It's the crucial divide between the two men. Biden embraces proposals he tacitly concedes could be viewed as incremental, because he doesn't think Sanders' ideas have any chance of becoming law. Likewise, he doesn't mind explaining past votes on measures that many Democrats now criticize and that Biden himself has moved left on.

For his part, Biden said he doesn't see a fundamental conflict between the candidates' goals for universal health care access, accessible higher education and a more even economy. "We disagree on the detail of how we do it. But we don't disagree on the principle," Biden said.

The question remains whether Sanders' progressive supporters accept that framing.

GRUMPY OLD MEN

The advanced ages of Biden and Sanders were often on display -- but nowhere more so than when they yelled at one another.

During an especially testy exchange, the former vice president tried to laugh off an accusation that he had once advocated on the floor of the Senate for cutting Social Security benefits. That prompted Sanders to proclaim, "Don't laugh, Joe!"

Then, as Biden defended himself on the same issue, Sanders instructed the viewing audience to check out "The YouTube" for archival footage of Biden doing what he claimed he'd never done. He later corrected himself, saying video of Biden arguing for cutting Social Security was "all over" YouTube.

Meanwhile, both candidates were quick to say they had not suffered any symptoms of coronavirus, but it didn't always seem that way. Biden cleared his throat loudly and frequently at the start of the debate, and the camera caught Sanders later wiping his nose with a tissue.

CLIMATE CHANGE CLASH

Sanders repeatedly chided Biden for not going far enough in his plans to combat climate change, but used a line of attack that could ultimately cost Democrats general election votes in the battleground state of Pennsylvania and Ohio.

The Vermont senator said that he would ban hydraulic fracking because of its deep damage to the environment. The former vice president said he too would move to slow the effects of climate change, but hasn't called for fully forbidding fracking. That prompted Sanders to say to Biden, "I know your heart is in the right place but this requires dramatic, bold action."

One of the reasons Biden might hesitate to go that far is how important fracking is the economy in Pennsylvania. Donald Trump's narrow victory there and in the states of Michigan and Wisconsin in 2016 helped propel him to the White House despite losing the national popular vote to Hillary Clinton. Democrats are desperate to take those states back from the president in November.

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Barrow reported from Atlanta.