This was the moment Joseph R. Biden Jr. had to know would be coming. He did not seem entirely prepared.
About an hour into the debate on Thursday, Senator Kamala Harris, the African-American former prosecutor from California, had edged into the cross-talk with a request: “I would like to speak on the issue of race.”
Soon, her precise intention was clear. She paused and took a breath. “I’m going to now direct this at Vice President Biden,” she said, turning toward the 2020 primary front-runner who waxed nostalgic earlier this month about working with segregationist senators.
With that remark, Ms. Harris brought to the fore the central questions threatening Mr. Biden’s tenuous perch atop the polls.
Will the Democratic Party of 2020 — buoyed last year by the midterm success of a new generation of female and nonwhite candidates — really choose as its standard-bearer a center-left white man who joined the Senate during the Nixon administration? And if not, who will be the contender willing to challenge him most directly, with so many voters chiefly concerned about uniting to defeat President Trump?
As Ms. Harris spoke, Mr. Biden’s head angled down, cocking to the left. He blinked repeatedly. He clutched both ends of the lectern like a flotation device.
Entering Thursday evening, Mr. Biden had subsisted as a peculiar kind of favorite: 76 and rusty, his speeches heavier on curious digression than stirring crescendo, unapologetic about his affection for a bygone era of comity. And that, to Ms. Harris, was precisely the problem.
“I also believe — and it is personal — and it was actually very 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,” Ms. Harris said, pressing her open right hand into her lectern. She cast Mr. Biden as a figure rooted in an entirely different era, citing his past opposition to busing.
“There was a little girl in California who was a part of the second class to integrate her public schools and she was bused to school every day,” Ms. Harris said. “And that little girl was me.”
Mr. Biden responded alternately with defensiveness and indignation. “I did not praise racists,” he said, also accusing her of mischaracterizing his position on busing. “That is not true.”
[We fact-checked the debate, including Mr. Biden’s defense of his record on busing.]
But Mr. Biden’s political vintage quickly showed in his answer, as he noted that he had worked as a public defender after the assassination of Martin Luther King Jr. in 1968. “I ran because of civil rights,” Mr. Biden said. “I continue to believe we need to make fundamental change.”
And after winding up for a longer defense of his record on race, Mr. Biden — the son of Scranton who served beside the nation’s first black president — stopped abruptly, appearing to surrender the exchange. “Anyway,” he said, “my time is up.”
In this one episode, the debate seemed to capture the race’s key tensions as the campaign enters a new, more combative phase where Democratic candidates will more aggressively seek to establish a contrast in the sprawling field.
[Mr. Biden found himself on the defensive throughout the night. Read our recap.]
Biden loyalists used to think he could talk his way through anything, recalling his marathon town hall forums as a first-time presidential candidate in 1988 or his relative reputational Teflon amid verbal slips as Barack Obama’s vice president, like the time he got out ahead of his boss in endorsing same-sex marriage — and only endeared himself further to many in the party.
But the opening months of Mr. Biden’s campaign have often felt like an exercise in disaster avoidance, with a highly cautious principal and decidedly mixed results. His every utterance has risked further alienating a progressive base that distrusts him. His references are almost ostentatiously outdated. Even supporters are unlikely to cite his policy feats as the main attraction, preferring to dwell on his unpretentious bearing, his grace through personal tragedies and his time in the Obama White House.
Mr. Biden has lurched through uncomfortable explanations of the less-than-liberal moments in his record, despite largely avoiding extended interviews or unrehearsed settings. Even under gentle inquisition on “The View” in April, Mr. Biden strained to weather predictable turbulence as hosts asked about his treatment of Anita Hill at the 1991 confirmation hearings of Justice Clarence Thomas. At times, he was reduced to cutting himself off midsentence as he abandoned a line of defense.
The challenge for Mr. Biden entering Thursday evening was presenting himself as a figure in step with the times while turning his wealth of experience to his advantage. Often, he struggled. Boasting of his skill at winning concessions from Senator Mitch McConnell, the immovable Republican majority leader, Mr. Biden suddenly found himself under fire from Senator Michael Bennet of Colorado, a mild-mannered moderate who depicted Mr. Biden’s deal making as a fiasco.
Taking aim at Mr. Biden’s negotiation with Mr. McConnell to partly repeal a tax cut from the George W. Bush era, Mr. Bennet appraised the arrangement as “a great deal for Mitch McConnell.”
That negotiation with the majority leader took place relatively recently. But Mr. Biden has been in public life for more than 40 years, a history that provided ample opportunities for others onstage to make an implicit case for him to step aside. “As the youngest guy on the stage, I feel like I probably ought to contribute to the generational conversation,” Pete Buttigieg, the millennial mayor of South Bend, Ind., interjected at one point.
[How did Sanders and Buttigieg do? Here are six takeaways from the debate.]
Of course, Mr. Biden is not the only candidate who would be over 70 on Election Day. But the others atop the field, Senators Bernie Sanders and Elizabeth Warren, have been at the vanguard of the party’s leftward tilt, sharpening their economic arguments to a fine point in recent years and making plain their urgency not just to remove and replace Mr. Trump but to address the root conditions that produced his election in the first place.
In a few important moments, Mr. Biden managed to jolt himself toward the future and outline a progressive agenda he would pursue as president. Having unveiled only a few signature proposals so far on the campaign trail, he pledged to take aggressive action to combat climate change, and to reinforce the middle class with new benefits like an optional government health care plan.
It remains to be seen, however, whether Democratic voters will embrace as an exciting vision a package of ideas that represent, in aggregate, much of the unfinished business of the Obama administration.
Mr. Biden’s strategy has been to float above the fray, to the extent possible, projecting the aura of a capable statesman who could restore a measure of calm to the office after one Trumpian term.
As he took the debate stage for the first time this primary season, several rivals were unwilling to let him coast. Ms. Harris said near the beginning of the evening that America “does not want to witness a food fight,” though her exchange with Mr. Biden over race was the sharpest of the night.
But even lesser-known candidates recognized the value in taking on Mr. Biden by name.
“I was 6 years old when a presidential candidate came to the California Democratic convention and said it’s time to pass the torch to a new generation of Americans,” Eric Swalwell, a long shot congressman, said early on. “That candidate was then-Senator Joe Biden.”
Mr. Biden gave a little smile and theatrically narrowed his gaze.
“Would you like to sing a torch song?” a moderator, José Díaz-Balart, asked.
“I would,” Mr. Biden said, eyes widening. “I’m still holding onto that torch, I want to make it clear to you.”
The line landed to scattered applause. But here was Mr. Biden’s chance to nail the next beat, to make the case that his time had not passed. He opted instead for an unsteady recitation of a general education platform (highlighting the need to “make sure that everybody is better prepared to go onto education — for an education”) that concluded with this: “We can’t put people in the position where they aren’t able to go on and move on.”