US President Joe Biden pledged to stay in the White House race Friday, defying a growing Democratic party revolt that raised speculation he could bow out as soon as this weekend.

"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 ten more House Democrats and two senators 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 four senators have now called on him to drop out.

A key donor, Silicon Valley investor Michael Moritz, also joined other supporters such as actor George Clooney who want Biden to make way.

"Sadly, President Biden has a choice -- vanity or virtue," the New York Times quoted Moritz as saying.

With reports that top Democrats have also expressed concerns, polls showing Trump 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 front-runner 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 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."