Joe Biden reassures supporters 'I'm OK' as Democrats remain split ...

Joe Biden is attempting to restablise his shaky election campaign, addressing concerns about his age in a speech to supporters.

Twenty Democratic members of Congress are now publicly calling on the president to exit the race for re-election, and donors have reportedly frozen $US90 million in funds.

Mr Biden is campaigning in Detroit, in the swing state of Michigan.

Democrats remain split over Joe Biden's candidacy for US president, with some piling on pressure to force him out of the election race as others push back and try to protect him.

A day after a prime-time press conference that gifted his opponents two highly shareable gaffes, the president is campaigning in the battleground state of Michigan. He has joked about his age with local supporters, and told them: "I promise you I'm OK."

"I'm only 41," he quipped as he spoke to an assembled group in a Detroit restaurant.

"For the longest time I was too young," Mr Biden said, "because I was the second-youngest man ever elected to the United States Senate.

"And now I'm too old.

"But hopefully, with a little bit of age comes a little bit of wisdom. And … in this moment, I think the alternative is not much of an alternative."

Mr Biden later spoke at a campaign rally in Detroit, walking onto the stage to chants of, "Don't you quit," from the crowd. 

"You've probably noticed there's been a lot of speculation lately," he said during an energetic and relatively stumble-free performance.

"I am running and we're going to win. I'm not going to change that."

Oscar-winning actor Octavia Spencer greeted Joe Biden in Michigan and spoke at his campaign rally.(AP: Jacquelyn Martin)

He also complained he had been taking a "hammering" for his errors while Mr Trump had been given a "free pass".

"People would rather talk about how I mix up names," he said. "I guess they don't remember that Trump called Nikki Haley 'Nancy Pelosi'." 

The Biden camp has pointed to polling conducted since last month's debate, as it attempts to restabilise an election campaign knocked off course by intensifying doubts about Biden's fitness for office.

The NPR/PBS NewsHour/Marist poll has Mr Biden leading Mr Trump, 50 per cent to 48 per cent in a head-to-head match-up (2 per cent of respondents were undecided.)

It is a slight improvement for Mr Biden on the previous month's poll, conducted before the debate, which found the men were tied on 49 per cent each.

However, the change is within the margin of error (of 3.3 per cent)

And when third-party candidates are introduced, Mr Trump edges ahead, 43-42. (Robert F Kennedy had 8 per cent, Cornel West 3 per cent, and Jill Stein 2 per cent.)

"Polling continues to show the same race we've been seeing, right? One that is close and unaffected by the debate," Biden campaign spokesman Michael Tyler said.

He said campaign donations "exploded" during the president's press conference on Thursday, local time. "We hit seven times our average."

But two more members of Congress joined the list of Democrats publicly pressing the 81-year-old president to move aside for a younger candidate.

The calls from Colorado's Brittany Petterson and California's Mike Levin mean there are now 19 House Democrats and one senator publicly asking Mr Biden not to run.

Ms Petterson called on Mr Biden to "pass the torch to one of our many capable Democratic leaders so we have the best chance to defeat [Republican candidate] Donald Trump". 

Mr Levin said he made the decision after hearing from hundreds of constituents in the fortnight since Mr Biden's debate against Mr Trump. 

"Making this statement is not easy," he said in a letter given to LA TV station KTLA.

The New York Times is reporting that major donors have meanwhile frozen $US90 million ($132 million) in pledged donations while Mr Biden remains the candidate.

Hakeem Jeffries, the most senior Democrat in the House of Representatives, said he requested a meeting with Mr Biden, which was granted after his major media conference yesterday.

"I directly expressed the full breadth of insight, heartfelt perspectives and conclusions about the path forward," Mr Jeffries wrote in a letter to colleagues after the meeting. He did not voice a personal view either way.

But some senior Democrats have expressed support.

California Governor Gavin Newsom, one of the Democrats often touted as a possible replacement for Mr Biden, said he had been responding to such suggestions with "delete, delete, delete, delete".

In an interview excerpt published by CBS, Mr Newsom said he was "all in, no daylight" as a supporter of Mr Biden.

And influential South Carolina congressman Jim Clyburn told NBC he was "ridin' with Biden".

Asked if he believed Mr Biden was "still the same man" as he was in 2020, Mr Clyburn said: "Physically, I don't think so. Mentally, I do think so."