REHOBOTH BEACH: Defying a growing Democratic party revolt that raised speculation Joe Biden could bow out as soon as this weekend, the incumbent president doubled down on his pledge to stay in the White House race Friday.

"The stakes are high, and the choice is clear. Together, we will win," the 81-year-old said in a statement from the Delaware beach home where he is in Covid isolation.

"I look forward to getting back on the campaign trail next week," Biden added, as his doctor said the president was bouncing back from the symptoms of the disease.

But Biden's political health appeared to be in far worse shape, with nine more House Democrats and one senator joining the list of lawmakers publicly calling on him to quit November's election clash with Donald Trump.

A disastrous debate performance against Trump three weeks ago triggered panic about Biden's age and health. More than 30 House Democrats and three senators have now called on him to drop out.

With reports that top Democrats have also expressed concerns, polls showing the Republican on course for a return to the Oval Office, and fundraising drying up, the walls appeared to be closing in.

NBC News reported that some of Biden's family had "discussed what an exit from his campaign might look like" although there was no final decision to do so.

Vice President Kamala Harris, the frontrunner to succeed him as the Democratic presidential candidate if Biden does drop out, was holding an emergency call with donors on Friday.

Biden's campaign however also pushed back against reports that he would bow out, saying that while there had been some "slippage" in support, he was still the best candidate.

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

'Dark vision'

Biden's statement blasted his rival's speech at the Republican National Convention in Milwaukee on Thursday, saying that "Donald Trump's dark vision for the future is not who we are as Americans."

But the split-screen between the two campaigns has been stark, with Trump receiving a triumphant reception from newly unified Republicans after surviving an assassination attempt on Saturday.

The pressure on Biden has soared in the last 48 hours, with reports that former president Barack Obama, ex-House speaker Nancy Pelosi and the party's congressional leaders had all expressed concerns behind the scenes.

Top House Democrat Hakeem Jeffries was evasive on Friday, saying that the "ticket that exists right now is the ticket we can win on" but saying it was Biden's "decision to make."

The scene could now be set for the most crucial weekend of Biden's presidency, with media speculation that the US leader is using his time hunkered down in Rehoboth Beach to consult family members and mull over the path ahead.

The NBC report said it could involve a "carefully calculated plan" to step aside based on his own timing, to give some dignity to what would be a historically late decision by a sitting US president not to run.

Any decision by Biden to step aside less than four months from the election would also have to try to avoid chaos in the Democratic Party over his successor as nominee.

Biden beat Trump in 2020, becoming the oldest president in US history in the process.

But a series of polls have shown him trailing Trump in the 2024 race despite his rival being a convicted felon, while some polls show Harris as more competitive.