Widespread concerns about President Biden’s age pose a deepening threat to his re-election bid, with a majority of voters who supported him in 2020 now saying he is too old to lead the country effectively, according to a new poll by The New York Times and Siena College.

The survey pointed to a fundamental shift in how voters who backed Mr. Biden four years ago have come to see him. A striking 61 percent said they thought he was “just too old” to be an effective president.

A sizable share was even more worried: Nineteen percent of those who voted for Mr. Biden in 2020, and 13 percent of those who said they would back him in November, said the 81-year-old president’s age was such a problem that he was no longer capable of handling the job.

The misgivings about Mr. Biden’s age cut across generations, gender, race and education, underscoring the president’s failure to dispel both concerns within his own party and Republican attacks painting him as senile. Seventy-three percent of all registered voters said he was too old to be effective, and 45 percent expressed a belief that he could not do the job.

This unease, which has long surfaced in polls and in quiet conversations with Democratic officials, appears to be growing as Mr. Biden moves toward formally capturing his party’s nomination. The poll was conducted more than two weeks after scrutiny of his age intensified in early February, when a special counsel described him in a report as a “well-meaning, elderly man with a poor memory” and “diminished faculties in advancing age.”

Previous polling suggests that voters’ reservations about Mr. Biden’s age have grown over time. In six top battleground states surveyed in October, 55 percent of those who voted for him in 2020 said they believed he was too old to be an effective president, a sharp increase from the 16 percent of Democrats who shared that concern in a slightly different set of swing states in 2020.

Voters have not expressed the same anxieties about Donald J. Trump, who at 77 is just four years Mr. Biden’s junior. Their likely rematch would make them the oldest presidential nominees in history.

If re-elected, Mr. Biden would beat his own record as the oldest sitting president, while Mr. Trump would be the second-oldest if he won. Mr. Trump would be 82 at the end of the term, and Mr. Biden would be 86.

Otto Abad, 50, an independent voter in Scott, La., said he voted for Mr. Biden in 2020 but planned to flip his support to Mr. Trump if they faced off again. Last time, he wanted a less divisive figure in the White House after the chaos of the Trump administration. Now, he worries that Mr. Biden is not quite up for a second term.

“If he was in this sort of mental shape, I didn’t realize back then,” Mr. Abad said. “He’s aged a lot. With the exception of Trump, every president seems to age a lot during their presidency.”

He added: “Trump, one of the few things I would say good about him, is that nothing seems to bother him. He seems like he’s in the same mental shape he was 10 years ago, 12 years ago, 15 years ago. He’s like a cockroach.”

Mr. Abad is far from alone. Just 15 percent of voters who supported Mr. Trump in 2020 said they thought he was now too old to be an effective president, and 42 percent of all voters said the same — a much lower share than for Mr. Biden. Polling from the 2020 race indicates that the share of voters who believe Mr. Trump is too old has also increased over the past four years, but not as drastically as for Mr. Biden.

In the most recent Times survey, 19 percent of all voters said Mr. Trump’s age was such a problem that he was not capable of handling the presidency. And in a sign of Republicans’ far greater confidence in their likely nominee, less than 1 percent of voters who backed Mr. Trump in 2020 said his age made him incapable.

Mr. Biden and his allies have rejected anxieties about his age and mental acuity as unfair and inaccurate. His campaign says its coalition will again rally around the president once it fully recognizes that Mr. Trump could win back the White House. It also argues that Mr. Biden faced age concerns in 2020 and still won.

Yet Mr. Biden is now four years older, and it may be impossible to completely reassure voters about his age given the inexorable march of time. The poll indicates that the worries about him are not only pernicious but also now intertwined with how many voters view him.

Calvin Nurjadin, a Democrat in Cedar Park, Texas, who plans to support Mr. Biden in November, said he was unconvinced by politicians in his party who have publicly played up their direct observations of Mr. Biden’s mental sharpness.

“You’ve just kind of seen the clips of, you know, he’s having memories onstage and, you know, during debate and discussion where he kind of freezes up a lot,” said Mr. Nurjadin, who does data entry work. “Him being sharp and fit is not very convincing.”

Even though the country is bitterly divided and Republican voters have overwhelmingly negative views of Mr. Biden’s age, Democrats do not appear to be more worried about the effects of time on Mr. Trump than on Mr. Biden. Similar shares of Democrats said each man was too old to be effective.

The poll tried to understand in greater depth how voters thought about Mr. Biden’s and Mr. Trump’s abilities. The survey first asked if each man was too old to be effective. Voters who said yes were asked a follow-up question about whether that age was such a problem that Mr. Biden or Mr. Trump was not capable of handling the job, a stronger measure that prompted voters to consider the candidate’s basic fitness for office.

Shermaine Elmore, 44, a small-business owner in Baltimore, voted for Mr. Biden four years ago, backing the Democratic candidate as he had in previous elections.

But he said he had made more money under Mr. Trump, blaming inflation and gas prices for his losses during the Biden administration. He planned to vote for Mr. Trump this fall.

Of Mr. Biden, he said: “I don’t think he’s in the best health to make a decision if the country needs the president to make a decision.”

Samuel Friday, 28, a database administrator and a Democrat in Goose Creek, S.C., said he planned to vote for Mr. Biden but had some apprehension about whether the president would survive a second term.

“In terms of his health, I think people have come out and said that he’s healthy as can be, which is always positive,” he said. “But when you get to a certain age, there is the higher risk that the president could die in office. And I’m not sure that Kamala Harris would be the choice that I would want in the presidency.”

Indeed, the vice president is not seen any more positively than Mr. Biden. Only 36 percent of all voters said they had a favorable view of Ms. Harris.

About two-thirds of those who voted for Mr. Biden in 2020 expressed a positive view of Ms. Harris, nearly the same as for the president. And in a head-to-head contest with Mr. Trump, Ms. Harris did not fare any better than Mr. Biden, losing by six percentage points.

While Democrats are still divided, they also seem to be slowly unifying behind Mr. Biden’s bid. Forty-five percent of Democratic primary voters said he should not be their party’s nominee, compared with 50 percent who expressed that view in July.

Margaret Stewart, a retiree from Westland, Mich., said she would have preferred a younger nominee but was not particularly bothered by Mr. Biden’s age. The president, she said, sometimes makes verbal missteps when he is stressed but is mentally fit to serve as president.

“Some of the little flubs he had, one, he’s had those forever,” she said, “and I honestly think his memory is better than mine when I was in my 40s.” She added, “He’s not senile.”

Overall, voters generally express warmer views about Mr. Biden than Mr. Trump. Fifty-one percent of registered voters said the president had the personality and temperament to be president, compared with 41 percent who said the same about Mr. Trump. Among Republicans, 27 percent said Mr. Trump lacked those traits, while 14 percent of Democrats said the same of Mr. Biden.

Brian Wells, 35, a lawyer from Huntsville, Ala., described himself as a reluctant supporter of Mr. Biden. He was frustrated that there were not other choices for the top of the presidential ticket, and was convinced that Mr. Biden was not entirely up to the duties of the office.

Still, Mr. Wells plans to cast his ballot to re-elect the president in November.

“He’s incompetent. He’s clearly struggling to fulfill his duties,” he said. “He’s clearly reached the point where he’s too old for the job. But he’s still a step ahead of Trump.”

Camille Baker contributed reporting.

The New York Times/Siena College poll of 980 registered voters nationwide was conducted on cellular and landline telephones, using live interviewers, from Feb. 25 to 28, 2024. The margin of sampling error for the presidential ballot choice question is plus or minus 3.5 percentage points among registered voters. Cross-tabs and methodology are available here.