On Thursday night, the former vice president grew defensive and rambled when Senator Kamala Harris confronted him in stark and personal terms on his record on race. Standing under the bright lights, as he haltingly told the second black woman ever elected to the Senate that his opposition to federally mandated busing did not affect her childhood, the former vice president did not exactly seem like the heir apparent to a party that has transformed around him during his decades in public life.

Despite his lead in the polls, last week’s debates revealed how difficult that could be for Biden.

MIAMI — Since he joined the presidential race in April, there has been one overarching question about Joe Biden: Can he convince millions of Democratic primary voters that he alone is the best bet, among a field brimming with candidates younger, more liberal, and more diverse than he, to be the standard-bearer for his party?

Biden, 76, has cultivated an image as a powerful front-runner with close ties to former president Barack Obama, but he is entering a tougher stage of the primary — one in which he may be forced to rethink his strategy of meeting critiques with defiance or a laugh, especially on charged issues such as race.


“I think he needs to remember that he’s running in 2019 and not 1988,” said Joseph Darby, a pastor from Nichols AME Church in Charleston, S.C., and an official with his city’s NAACP. “The political discourse is a little bit different now,” said Darby, who is neutral in the primary and called both Biden and Harris impressive. “I think he’s still coming up to speed with that.”

Harris’s surgical and carefully planned attack laid bare the differences between Biden and many of his competitors — in age, life experience, and the way they believe Democrats should talk about race, a defining and unavoidable issue for many of the voters who will choose the party’s nominee in 2020. Biden has longstanding relationships with black leaders, but he does not seem used to the careful tone that may be required of an older white politician in a party that now has people of color decades younger than he is also seeking the presidency.


Biden didn’t appear prepared for Harris’s slam on his past work with segregationist senators to prevent federal support for busing, despite the fact that his praise of working with those senators had dominated the news for the week leading up to the debate.

He told Harris she was wrong about his position, took a swipe at her career as a prosecutor — a critique that did not seem to land in the moment — and then cut himself off as he enumerated his civil rights record. “My time is up,” he said. “I’m sorry.”

Biden’s attempts to defend himself made for a sharp contrast with the way Mayor Pete Buttigieg of South Bend, Ind., responded on the same stage when he was asked why there are so few black police officers in his city, which is reeling after a white police officer fatally shot a black man there this month.

“Because I couldn’t get it done,” Buttigieg, 37, said.

It’s unclear if the lackluster performance will eat away at the former vice president’s commanding 15-point lead in the polls, or whether Harris’s pointed criticisms of Biden’s record on race will siphon away his deep well of support from older black voters, many of whom find his eight years serving under the nation’s first black president deeply compelling.


“They’re going to forgive a lot of things and they’re going to forget a lot of things,” said Bill Clyburn, an African-American state representative from South Carolina who has endorsed Biden.

“They’re going to do that because of the confidence that they have in Biden and the confidence they have in Barack, who chose him,” added Clyburn, who is a cousin of the powerful Congressman Jim Clyburn.

Indeed, some of the controversies that play out in the news media don’t appear to register on the campaign trail, as when Biden joked about accusations from some women that his physical campaigning style made them uncomfortable. Many voters appear to trust him when he suggests his heart is in the right place on race, and appreciate his unstudied style.

“I think he’s a straight shooter, he’s very sincere, even in his colloquial conversation,” said Lois Ayer, 72, a retired teacher who saw Biden speak in Nashua last month.

But for months, operatives in early voting states have worried Biden is too reliant on the sheen from his role as vice president in the Obama administration in a wide-open primary where the party is wrestling with fundamental questions about identity and values. Biden keeps a lighter schedule than many of his rivals, has done fewer in-depth interviews with the press, and often reacts defensively when presented with criticisms of his comments or past positions on issues.


“They’ve been acting sort of aloof and like he’s stratospherically above everyone else, but I don’t think that’s sustainable for much longer,” said Brian Fallon, a former top aide to Hillary Clinton’s 2016 campaign.

Harris moved quickly to capitalize on her clash with Biden, selling T-shirts that commemorate the moment and announcing on Saturday she had raised $2 million in the 24 hours after the debate.

Biden initially acted like a front-runner with no desire to quickly clean up the mess. As his competitors grabbed more camera time post-debate, Biden had to be chased by a TV reporter through the debate hall. He reiterated his belief that Harris mischaracterized his views, and has since drawn a distinction between voluntary busing and busing mandated by the Department of Education, although as recently as his 2007 memoir he had called busing a “liberal train wreck.” On MSNBC, one of his aides said many African-Americans at the time believed busing was “not the best way” to integrate schools.

Then on Friday, in a speech at a labor luncheon for a civil rights group in Chicago, Biden took a softer tone in defending his commitment to racial equality.

“I heard and I listened to and I respect Senator Harris,” he told the crowd. “But we all know that 30 seconds to 60 seconds on a campaign debate exchange can’t do justice to a lifetime commitment to civil rights.”


But another comment he made at the same event, about how a “kid wearing a hoodie” could be the next poet laureate and not a “gangbanger,” drew a fresh round of criticism, underscoring the scrutiny Biden is likely to face when he discusses race in the coming weeks of his campaign.

New Jersey Senator Cory Booker, who is black, posted a retort on Twitter. “This isn’t about a hoodie. It’s about a culture that sees a problem with a kid wearing a hoodie in the first place,” said Booker, another Democratic contender who has been sharply critical of Biden on racial issues. “Our nominee needs to have the language to talk about race in a far more constructive way.”

Adrianne Shropshire, the executive director of BlackPAC, a progressive group that funds efforts to mobilize black voters, said Harris’s deft highlighting of Biden’s associations with segregationists could prompt some black voters to take another look at his record on race. She suggested Biden reckon with the issue directly.

“He needs to either make a major policy speech or in some other way articulate his evolution on issues,” Shropshire said. “He needs to discuss why he took certain positions that frankly were problematic.”

But such a move would require Biden to curb his instincts to dismiss criticism out of hand.

The former vice president has been prickly on the campaign trail, explicitly refusing to apologize after critiques of issues from his past treatment of Anita Hill when she accused Supreme Court nominee Clarence Thomas of sexual harassment, to his minimalist campaign schedule. He also punches down, responding to barbs from minor rivals and thus elevating the controversy, as when he slammed tech executive Andrew Yang for smack talking Biden’s decision to skip a candidate forum.

And last week, when Booker slammed Biden for joking about a segregationist senator using the word “boy,” Biden told reporters Booker should apologize to him, not the other way around.

Biden has survived a long career in politics, and facing down criticism aggressively is not always a political liability — he appears to have put the controversies around his physical style of campaigning to rest by mocking the issue, for example.

“I think a lot of people saw value in that strategy like, ‘Oh, OK, he’s not going to give into this wokeness trend of the Democratic Party,’ ” Fallon said. “But when he handles all of them the same way, there’s going to be a cumulative effect that wears on people.”

Jess Bidgood can be reached at Jess.Bidgood@globe.com. Follow her on Twitter @jessbidgood. Liz Goodwin can be reached at elizabeth.goodwin@globe.com. Follow her on Twitter @lizcgoodwin