A defiant President Joe Biden vowed on Wednesday to keep running for re-election, rejecting growing pressure from Democrats to withdraw after a disastrous debate performance raised questions about his readiness.

But in an ominous sign for the president, a leading ally publicly suggested a way that the party might choose someone else.

“I am running. I am the leader of the Democratic Party. No one is pushing me out,” Mr Biden said in a call with staffers on his re-election campaign, according to a top aide who posted his comment on the X social media platform.

Mr Biden was pulling every possible lever to try to salvage his re-election campaign – talking to top legislators, pumping up his campaign staff and meeting later in the day with Democratic governors before a planned weekend blitz of travel and a network TV interview.

But there were signs that support for Mr Biden was rapidly eroding among Democrats on Capitol Hill.

Representative Jim Clyburn, a long-time Biden ally, said he would back a “mini-primary” in the run-up to the Democratic National Convention next month if Biden were to leave the race.

Mr Clyburn, of South Carolina, floated an idea that appeared to be laying the groundwork for alternatives by delegates during the Democrats’ planned virtual roll call that is scheduled before the more formal party convention.

“You can actually fashion the process that’s already in place to make it a mini-primary and I would support that,” Mr Clyburn told CNN.

He said vice president Kamala Harris, governors and others could join the competition. “It would be fair to everybody … because if she were to be the nominee, we need to have a running mate. And need a strong running mate.”

Mr Clyburn, a senior lawmaker who is a former member of his party’s House leadership team, said he has not personally seen the president act as he did on the debate stage last week.

“I saw what I saw last Thursday night, and it is concerning,” he said.

Some suggested Ms Harris was emerging as the favourite to replace Mr Biden if he was to withdraw, although those involved in private discussions acknowledge that governors Gavin Newsom of California and Gretchen Whitmer of Michigan remain viable alternatives. But for some insiders, Ms Harris is viewed as the best prospect to quickly unify the party and avoid a messy and divisive convention fight.

Even as pressure around Mr Biden mounted, he and Ms Harris made a surprise appearance on a call with staffers from his re-election campaign and offered a pep talk, stressing the stakes of the election and returning to Mr Biden’s previous post-debate comments that he would get back up after being knocked down.

The president told those assembled that he was not leaving the race and would not be dragged out.

Asked a short time later whether Mr Biden would consider stepping down, White House press secretary Karine Jean Pierre said “absolutely not”.

“He understands it is fair for people to ask that question.” she said, while adding, “I cannot lay out something that would change the president’s mind” about seeking a second term.

She also said Mr Biden “is clear-eyed. And he is staying in the race.”

Democrats are unsatisfied with the explanations of Mr Biden’s debate performance, from both White House staff and the president himself. And there is a deeper frustration among some Democrats who feel Mr Biden should have handled questions about his stumbling debate performance much sooner and that he has put them in a difficult position by staying in the race.

White House chief of staff Jeff Zients urged people during an earlier all-staff meeting on Wednesday to tune out the “noise” and focus on the task of governing.