He won the most votes of any US presidential candidate ever last time around, but even many supporters feel unsure about sending the 80-year-old Joe Biden back to the White House for another term.

"He's just a little old," said Laura Miranda from New Jersey.

Biden is already the oldest-ever US president. He would be 86 by the end of a second stint in the job.

"But I don't know who else it would be," the 31-year-old said, referring to the dearth of nationally popular Democratic Party possibles.

"He may be in good health now, but who knows, in six years' time how he's going to be," said voter Steven Hjupp from Connecticut.

"It's a very bleak situation," he said of Democrats' 2024 efforts against possible contenders such as former UN ambassador Nikki Haley, Florida governor Ron DeSantis -- and Donald Trump, who has already announced his own bid. 

Even voters who approve of Biden's job performance over the last two years -- a period encompassing Covid, the war in Ukraine, inflation and abortion rows -- expressed concerns about giving him another four years in the White House.

"Biden is, I don't know, 100, 120 years old," joked James Everett Newman, a firefighter in Houston, Texas. "I wish we could have a younger viable candidate for president."

"At the same time, we don't have that many younger alternatives because the Democratic Party has done a poor job cultivating younger leadership," the 34-year-old added.

"I like what Biden's done. I think he's done a really good job," said 72-year-old Roger Tilton in Washington.

Still, the retiree thinks it's time for Biden to move on. 

"I personally wish he wouldn't run again," Tilton said.

"Mentally, I'm certainly not as acute as I was 20 years ago, and there's no way he is either."

First-time voter Avery Gonzales said Biden was "probably a little antiquated" for the job.

"Maybe we should have some younger blood running things, rather than just old people," the 19-year-old told AFP in Los Angeles.

'Fresh ideas'

Others say Biden's age should not disqualify him if voters think he is otherwise fit for the job. 

"As long as you are healthy, and you are able to comprehend and use your head like you're supposed to, age is not a factor," said New York Biden supporter Rodney Grimes, 59. 

Aisha Smith, a 42-year-old in Houston, said she thinks Biden is brave to run for president again at his age, pointing out that there are elderly members of Congress and Supreme Court justices -- some are in their 70s and 80s. 

"I think age is only a number," she said.

For Jemima Homawoo, who was out walking near the White House Tuesday morning, if Biden "feels like he's able to do the job, then I think it's perfectly fine."

But the 33-year-old contractor still has reservations. 

"I would prefer, honestly, if an opportunity was given to someone who is a bit younger and may have fresh ideas for the country," she said. 

Scott Shinderman, 52, told AFP in Los Angeles that the prospect of a Trump-Biden rematch showed there was a certain amount of what he called sclerosis in the US political system, but added he would pick Biden at the ballot box, despite concerns over his advancing years.

"You can't be ageist, obviously. But...he's not as sharp as he used to be. That's evident in the way he speaks. 

"I don't know if that affects his decision making. As long as you have good people around you, sometimes that's good enough."

For Republican supporters, feelings on Biden's candidacy are much more clear-cut.

"I think it’s a terrible idea," said Riva Fernandez, in the beach town of Fort Lauderdale, Florida.

"I really hope DeSantis wins," the 54-year-old said. "He's the man."