US President Joe Biden is "absolutely" staying in the race for the White House, his campaign chair said Friday, despite mounting pressure from Democratic allies to step aside over worries about his age and health.

Jen O'Malley Dillon acknowledged there had been some "slippage" in support since the 81-year-old's disastrous debate performance against Donald Trump, but insisted he would still win in November.

"Absolutely the president's in this race," Jen O'Malley Dillon told MSNBC's Morning Joe program. "Joe Biden is more committed than ever to beat Donald Trump."

She added: "You have heard from the president directly time and again, he is in this race to win, and he is our nominee, and he's going to be our president for a second term."

Biden is currently holed up at his beach house in Delaware self-isolating with Covid.

Senior Democrats including former president Barack Obama, former House speaker Nancy Pelosi and the party's current leaders in the Senate and House have all reportedly expressed concerns that he will lose the election.

Twenty Democrats in the US House and two senators have meanwhile called on him to step aside from the race.

O'Malley Dillon, Biden's former deputy chief of staff, admitted that the campaign had faced a difficult time since he appeared tired and confused in the June 27 debate.

"I'm not here to say that this hasn't been a tough several weeks for the campaign," she said.

"There's no doubt that it has been, and we've definitely seen some slippage in support, but it has been a small movement."

A series of recent polls have shown Biden behind Trump in the 2024 race, including in all key battleground states.