WASHINGTON — Former Vice President Joseph R. Biden Jr. is 76 years old and prone to sometimes-rambling monologues with antiquated references. But if his age and missteps are supposed to be liabilities, they haven’t helped rivals who have tried to use them to their advantage.
Julián Castro, the former housing secretary, was the latest to learn that the hard way, taking direct aim at Mr. Biden’s age, and memory, in last Thursday’s debate. It didn’t work for Mr. Castro — amid a backlash, he lost a key endorsement over the weekend — and it hasn’t worked throughout the 2020 campaign.
While younger candidates have presented themselves as agents of generational change, it is three septuagenarians who have emerged as the front-runners: Mr. Biden, 78-year-old Senator Bernie Sanders of Vermont and 70-year-old Senator Elizabeth Warren of Massachusetts. They have generated the most support and enthusiasm, and taken their place in the top tier of the polls as they seek to challenge a president who is 73 himself.
And neither their age, nor Mr. Biden’s sometimes unsteady speaking performance, has convinced voters to get behind other candidates perceived as untested on the national stage.
“I ran a generational campaign that wanted to take on issues that were affecting my generation and future generations,” said Representative Eric Swalwell of California, who centered his presidential campaign around gun violence and student debt before withdrawing in July. “Those issues are really important, but beating Donald Trump is the most important thing that most voters want. I just sensed that they weren’t going to roll the dice on a generational candidate.”
The debate last week produced further scrutiny of age as a factor in the primary. Within days, Mr. Biden, Mr. Sanders and Ms. Warren were all asked whether they would release their medical records — and said they would. Even a hoarse voice that forced Mr. Sanders to cancel some events Monday and Tuesday was subject to some age-related speculation.
Mr. Biden’s debate performance left him once again open to questions about his age and acuity. He bungled the name of Moms Demand Action, the nation’s leading gun control group. He answered a question about institutional racism with a rambling tangent that included the suggestion that parents “make sure you have the record player on at night.” During the July debate, Mr. Biden forgot whether he was directing voters to his website or signing them up to receive text messages from the campaign.
Some voters have noticed. “I want Joe Biden to be able to complete a sentence better than I can,” said Melanie Gee, 58, a retired antiques dealer who attended a women’s event in Birmingham, Ala., on Monday. “I don’t think he’s senile. I just think he’s not as a sharp as I want him to be.”
Yvette Simpson, the chief executive of Democracy for America, a liberal group, said she worried about Mr. Biden’s ability to “deliver a clear message at a time when a clear message is important to the American people.’’
But while Mr. Biden’s missteps have at times put him on the defensive, his rivals have struggled to frame them in a way that diminishes him and enhances their own candidacies. Sometimes it just looks meanspirited.
Mr. Castro took the starkest shot yet at Mr. Biden’s capabilities during a heated exchange over health care policy at the debate. “Are you forgetting already what you said just two minutes ago?” Mr. Castro said, repeating himself three times. It had the effect of thrusting out into the open a fact that has been the subtext of other candidates’ strategies for months.
Later that night, Senator Cory Booker of New Jersey also expressed reservations about Mr. Biden, telling CNN that “there are definitely moments when you listen to Joe Biden and you just wonder.”
The attacks failed. By Sunday, Mr. Booker had backed off his Biden criticism — not the first time he’d done so during the campaign.
And Representative Vicente González of Texas defected from supporting Mr. Castro to Mr. Biden, in part because of Mr. Castro’s provocative questions on the debate stage, according to people familiar with his decision.
“If you’re polling in the low single digits and you’re not raising any resources and you’re fracturing your party and you’re just getting your supporters to be upset at other candidates, it certainly can’t be a good thing for our party,” Mr. González said Sunday on CNN, alluding to Mr. Castro’s attack.
At the same time, Mr. Biden’s defenders become protective of him when pressed with questions about his age. Former Senator Barbara Boxer of California, who is neutral in the 2020 primary, said criticisms of Mr. Biden’s age are “ageist.”
“He started off very strong and as it went on, he was trying to get in too many things, too many facts,” Ms. Boxer, 78, said of Mr. Biden’s debate performance. “It’s very hard standing there for three hours. I don’t care if you’re 55 years old.”
“That is just political attacks, right now people trying to figure out any way to try to knock the front-runner down,” Senator Doug Jones of Alabama said Sunday after attending a church service with Mr. Biden, whom he backs. “Joe’s going to be Joe. And that’s why people love him, and they know his history.”
There has been a clear recalibration among the second-tier candidates, who seem to have realized that drawing clear age contrasts with Mr. Biden — as Mayor Pete Buttigieg of South Bend, Ind., had been doing since his campaign began — is a losing strategy.
And not all of Mr. Biden’s younger rivals are eager to stress their age difference. “Who the hell cares,” Beto O’Rourke said Sunday on “Meet the Press.” “The last thing I care about is Joe Biden’s age.”
For Mr. Biden, no concern is raised more consistently by voters than his age, a subject Mr. Biden himself has said is fair game. When asked by reporters for his response, he often instructs voters to watch him in action on the trail, where he has made shows of physical force by sprinting through parades or lingering late into the evening to shake hands.
In July, Mr. Biden’s doctor said that he was in “excellent physical condition.”
Black voters, who comprise a critical bloc of Mr. Biden’s support, are not punishing him for his age, said Adrianne Shropshire, the executive director of BlackPAC, a super PAC focused on African-American Democrats. The black primary electorate skews older and is sympathetic to Mr. Biden’s age, she said.
“A lot of the age critique centers on cultural issues, record players and texting, which many of those voters struggle with as well but don’t see as disqualifying,” Ms. Shropshire said.
Mr. Sanders, who is 15 months older than Mr. Biden, and Ms. Warren do not face the degree of scrutiny about their age that Mr. Biden does, even as some voters at their events admit they would like to see someone younger elected president.
Mr. Sanders’s events are high-energy affairs, punctuated by the senator frequently waving his arms to emphasize his points. It also helps that he rarely deviates from themes in his stump speech, affording him far fewer opportunities to make off-the-cuff gaffes.
Still, he lost his voice during a campaign swing through Iowa and Colorado last week — a fact that became evident to millions watching last week’s debate.
“I kind of lost my voice somewhere in Colorado, and I hope I don’t have to go back there right away to find it again,” he joked Saturday in Las Vegas.
Some on the Sanders team have acknowledged privately that his raspiness is not his best look. The Sanders campaign cut short a planned swing through South Carolina on Monday and Tuesday. The campaign said he was “feeling great” but would return home to Vermont to “rest his vocal cords.”
Among the three candidates in their 70s atop the polls, Ms. Warren is the most crisp when speaking to crowds and fielding questions on the debate stage.
“The medical records will be there, and anybody’s welcome to take a look at them,” she said. “And what I’m hoping is, we’re going to get to do a lot more town halls between now and then, and more after that.”
If Ms. Warren wants her town halls and public appearances to speak for themselves on her stamina, she is likely aware that her entrances and exits are part of the case. She sprints as she enters campaign events. A video of her jogging across a grassy field at an event in New Hampshire last month went viral, and she was even caught on camera running into Penn Station in New York to catch a train earlier this year.
“Age is a figure of speech,” said Harry Reid, the former Senate Democratic leader, who is 79. “I think it’s an exaggeration to start crossing people off the list because they’re 70 years old.”
Katie Glueck and Lisa Lerer contributed reporting from Birmingham, Ala., Sydney Ember contributed reporting from Las Vegas and Thomas Kaplan contributed reporting from Washington.