WASHINGTON President Joe Biden said "I'm not going anywhere" as he faced calls by many Democrats to end his re-election bid, using the Fourth of July celebrations on Thursday to hit back at doubts about his stamina and mental acuity to continue his campaign.

The 81-year-old Democrat's shaky showing at a June 27 debate with Republican rival Donald Trump means his every appearance is now closely scrutinised. Many Democratic voters are worried about whether he can keep up a gruelling pace of work for the next 4 1/2 years and some in his party have urged him to step aside.

Biden was hosting the annual US Independence Day festivities at the White House on Thursday, including a barbecue for a few thousand active-duty military service members and their families.

Biden, in a suit with no necktie, began his remarks with a forceful “Happy Independence Day!"

Reading from a teleprompter, Biden made no major errors in delivering brief remarks, but at one point appeared to go off script to make reference to a war cemetery that Trump declined to visit while in office.

"By the way, you know, I was in that World War One cemetery in France. The one that one of our colleagues, a former president didn't want to go...," he said, his voice dropping to a low volume and trailing off.

"I probably shouldn't have said, anyway," Biden added, before continuing his remarks.

As Biden mingled and took selfies with guests, someone called out for him to "Keep up the fight."

"You got me, man. I'm not going anywhere," Biden said, repeating his pledge to remain in the race despite the growing calls to step aside.

Abigail Disney, granddaughter of Walt Disney who founded the company that bears his name and who has been a major Democratic donor, became the latest donor to call for Biden to withdraw from the presidential election, telling CNBC on Thursday that she will halt donations to the Democratic Party until he does so.

Vice President Kamala Harris is the leading contender to take his place in the Nov 5 election if Biden were to drop out, sources have said, though his allies believe he can assuage the concerns of voters and donors.

Among the events on Biden's calendar being closely scrutinised is an interview with ABC News on Friday that will be aired in full at 8pm ET (0000 GMT Saturday). He also travels to Wisconsin the same day for a campaign rally.

Dozens of Democrats in the House of Representatives are watching closely and prepared to ask Biden to step aside if he falters in the ABC interview, a source told Reuters. Democrats see capturing control of the House in November as critical, as it could be their last hold on power in Washington if Trump returns to the White House and Republicans capture the Senate.

Biden faces a new reality since last week's debate - even if he doesn't falter verbally or physically, serious concerns about his viability as a candidate are likely to linger. If he mangles words or looks unfocused or confused, he will face renewed pressure to depart.

If reelected, Biden would be 86 at the end of a second term. He is being asked by some former supporters to step aside to preserve his legacy and lessen the chances of a second Trump presidency. With just four months to go before the election, a decision needs to be made soon, they say.

Democrats, including top allies, have left the door open to having Harris at the top of the Democratic ticket.

The White House has repeatedly said the president was suffering from a cold and jet lag on the night of the debate. On Wednesday, White House Press Secretary Karine Jean-Pierre said Biden had not had any kind of medical exam since his annual physical in February.

"He did not get checked out by the doctor. It's a cold, guys. It's a cold," she said at a news briefing.

However, spokesperson Andrew Bates said on Thursday that Biden saw a doctor after the debate. "Several days later, the president was seen to check on his cold and was recovering well,” he said.

Trump, 78, who made multiple false statements from the debate stage in Atlanta, falsely claimed in a video that was circulated on social media that he had driven Biden out of the race. He made disparaging comments about Harris in the same video.

Asked in a radio interview with WURD that aired on Thursday morning, whether there was any reason for the American people to be concerned after last week's debate, Biden demurred.

"No, I had a bad debate," he said, adding that this should not erase what he has done as president for three and a half years.

Biden's standing in opinion polls took a hit after the debate. Some 59% of Democrats responding to a Reuters/Ipsos poll said that Biden was too old to work in government, a concern that has shown up persistently in public opinion polling over the past year.

  - Reuters