MIAMI — He had to have seen it coming. Yet Joe Biden seemed unprepared and tentative in defending himself from Sen. Kamala Harris’ blistering critique of his record on race during Thursday’s Democratic presidential debate.

To his detractors, the viral scene and stumble at the first big event of the Democratic primary undermined Biden’s biggest perceived strength: electability. This from the front-runner whose central argument is that he is the Democrat best suited to take on President Donald Trump one-on-one.

The controversy over Biden’s recent boast of his ability to work well with segregationist senators 40 years ago and his past opposition to forced busing showcased some of his weaknesses: his age, 76; his propensity for gaffes; and the argument that he is behind the times in a party that places increasing emphasis on diversity and equality over old-school Senate collegiality.

A day after one rival on the stage said it was time for Biden to “pass the torch,” U.S. Rep. Greg Meeks (D., N.Y.) compared him to an aging Muhammad Ali.

“He wasn’t the same at the end of his career as he was at the beginning of his career,” Meeks, on CNN, said of the legendary boxer.

And yet Biden supporters noted that it was just one debate, and the polls show his lead is built in part on his bond with African American voters after eight years as Barack Obama’s vice president.

“His answers to that question weren’t good,” said former Gov. Ed Rendell, referring to the exchange with Harris, but he argued that later debates will carry more weight.

“I know he’d be the best president. I know he’d do a good job with Donald Trump,” Rendell said. “But if over the course of time he demonstrates that he’s not prepared or not strong enough to hold up, I would reassess it — but I think there’s no reason to panic now.”

The seeds of the moment were planted when, at a fund-raiser June 18, Biden waxed nostalgic about finding ways to cooperate with segregationist Democratic senators in the 1970s. He said the point was to show that he could get things done even with people with whom he sharply disagreed.

He was widely blasted for the remarks, and the problem flared up in Thursday’s debate, the first national event of the primary, when the former vice president faced live attacks after weeks of carefully managed, largely scripted appearances.

Harris seized her moment, noting that she was the only black person on stage.

Beginning by saying she doesn’t think Biden is racist, the former federal prosecutor continued, “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. And it was not only that, but you also worked with them to oppose busing.”

She added, “There was a little girl in California who was part of the second class to integrate her public schools, and she was bussed to school every day. And that little girl was me.”

Her campaign tweeted out a photo of a young Harris, showing they were ready for the moment, even if Biden seemingly was not.

Among pundits and the media, it was the standout moment in two nights of debates, and Biden staggered.

“I did not praise racists. That is not true,” he said, before jabbing Harris by saying he was a public defender and not a prosecutor, and has a long history of supporting civil rights.

But he seemed to wilt as his answer continued, and eventually stopped mid-sentence.

“Anyway, my time is up,” he said, “I'm sorry.”

All of it underscored that Biden has not run a national campaign in more than eight years, has shown signs of rust — especially compared to his fresher rivals — and has acted in some ways like a candidate relying on his history and reputation as if the nomination were already his to lose.

After the debate, Biden was one of very few candidates who declined to come to the “spin room” to face further questions from reporters. Aides and surrogates went in his place.

“He’s the front-runner, everybody’s going to come after the front-runner, that’s the way it happens in politics,” said U.S. Rep. Cedric Richmond (D., La.), Biden’s campaign co-chair. “We knew it would be that type of night, it was exactly what it was.”

On Friday, Biden brought up the Harris exchange with members of the Rainbow Coalition, a civil rights organization founded by Jesse Jackson.

“I heard and I listened to and I respect Sen. Harris, but we all know that 30 seconds to 60 seconds on a campaign debate exchange can’t do justice to a lifetime committed to civil rights,” he said.

In Philadelphia, where enthusiastic supporters for other candidates gathered at debate watch parties, none was held for Biden. Several residents who tuned in, while skewing younger and more liberal, called him out of touch or noted his age.

“I’m from Delaware and I love him, but for me, Joe just seems two steps behind where we are as a party,” said Matt Harker, who watched the debate at a Pete Buttigieg party at Tabu Lounge & Sports Bar in Center City. “I wish we could take his scrappy Scranton-ness and put it into someone younger. I think he could absolutely do the job, I just don’t know if he has the appeal.”

Harker, 41, said his ideal candidate is a mix of character, policy, and the ability to “wipe the floor with Trump” in debates. “I don’t know if I see that in him. I see it in Harris,” he added. “She’d kick his butt.”

At the First Unitarian Church, where the Philadelphia chapter of the Democratic Socialists of America gathered, the debate watch party was as much anti-Biden as it was pro-Bernie Sanders. As the night wore on in the hot viewing room — and people had knocked back a few beers — expletives were shouted at Biden when he appeared on screen, and at one point a “Lock him up!” chant went up.

“He just feels incredibly divorced from what’s happening in the country right now,” Brittany Quinn said of Biden. “His proposals feel very status quo.”

The largely white, liberal Center City-ites were never going to be Biden’s base, though, said David O’Connell, an assistant professor of political science at Dickinson College in Carlisle, Pa. And, he said, they are more tuned into the debates than the majority of Democrats right now. “If you want more liberal voices, there’s 10 to 15 candidates to choose from. I think Biden’s actually pitching himself to the majority of Democrats, and they may not be as loud on social media but they’re there.”

Biden’s most vocal supporters frequently nod to pragmatism more than excitement: Donors, party elders, and even some people at his rallies often describe Biden as the safe bet for beating Trump, especially in swing states such as Pennsylvania, Michigan, and Wisconsin.

“Last night was one question in one debate in June 2019 with the convention a year away,” texted Alan Kessler, a Biden fund-raiser and Center City law partner. “Too much time to panic and no reason to.”

Neil Oxman, a longtime political campaign consultant in Philadelphia, said Harris’ moment showed one of Biden’s weaknesses — presentation on a national stage.

“Look, for those of us in this media market who’ve seen him for 40-plus years, he’s not a 30-second, clear, concise guy. He’s a rambling, takes-three-minutes-to-answer guy, where he goes in circles and commas and brackets.”

The key question, though, is whether any of it makes a difference in voter opinion of Biden, particularly whether it moves black voters who are seen as essential to his campaign.

Biden has had steady support from black elected officials, including many who defended his comments about working with segregationists, and on Friday he announced an endorsement from Atlanta Mayor Keisha Lance Bottoms — a sign of continued establishment backing.

Rendell argued that Harris, despite strong media reviews, will suffer more long-term political damage for first signaling that she supports eliminating private health insurance and then walking back her answer after the debate, saying she misunderstood the question. And he said no one believes Biden is racist or does not support civil rights, especially after the vice president’s work with Obama.

When it comes to debate fallout, Rendell said, “ask me that question Feb. 1,” just before the Iowa caucuses.