MIAMI — In his first face-off with his rivals for the 2020 Democratic nomination, Joe Biden came off as a candidate trapped in amber.

The former vice president has led in all the polls based on a presumption that he is the party’s strongest bet to beat President Donald Trump. Biden betrayed the weakness of that premise with a painfully feeble performance in Thursday night’s debate.

It wasn’t just that his age and his rustiness showed, though they did. The real problem was that Biden seemed to have been dropped onto the center of the debate stage from a different era and looked bewildered at times for having found himself there.

He indignantly defended the record he has built and the values he has displayed over nearly half a century in public life, while showing lapses in his grasp of the fact that the country has moved forward.

It was mystifying that Biden did not appear to have anticipated or prepared for attacks from opponents who share his views on most issues. Sen. Kamala Harris, D-Calif., was the most skillful at exposing Biden’s vulnerabilities, particularly on race, which could undercut his standing with African Americans who have warm feelings about the eight years he spent as Barack Obama’s vice president.

She told Biden that, to many like herself, his recent, ill-considered boasts about being able to work with segregationist senators in decades past stirred no fond memories of a time when even adversaries could find common ground. “It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country,” Harris said.

But the real affront, she added, was in Biden’s own actions. “You also worked with them to oppose busing,” Harris said. “And you know, there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me.”

Biden compounded the damage with a meandering defense echoing one of the arguments that defenders of segregation themselves often used: that he did not oppose integration but merely wanted to assure decisions from Washington would not override local control of education.

“Well, there was a failure of states to integrate public schools in America,” Harris shot back. “So that’s where the federal government must step in. That’s why we have the Voting Rights Act and the Civil Rights Act.”

Harris was not the only one to put a dent in Biden’s image as an elder statesman and conciliator. When Biden lauded himself for his ability to persuade Senate Majority Leader Mitch McConnell, R-Ky., to raise the top tax rate in 2012, Sen. Michael Bennet, D-Colo., swooped in. “The deal that he talked about with Mitch McConnell was a complete victory for the tea party,” Bennet said.”

Nor did Biden offer much of a defense when Obama’s record of deporting 3 million illegal migrants was raised, except to say: “President Obama, I think, did a heck of a job. To compare him to what this guy (Trump) is doing is absolutely, I find, immoral.”

A single debate, even one as disastrous as we saw on Thursday night, will not seal Biden’s fate. But it might have recalibrated expectations. His strategy thus far has been to hold himself apart from the fray and keep his focus on maintaining his viability as a choice in the general election against Trump.

Now, however, the fray has come to him, and he can no longer hold it back. A double onus is on Biden — to come to a more honest reckoning with his past and to outline a vision of the future that manages to be both convincing and inspiring.

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Karen Tumulty is a Washington Post columnist. Leonard Pitts is on vacation.