WASHINGTON, July 4 (Reuters) - 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 scrutinized. Many Democratic voters are worried

about whether he can keep up a grueling 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 U.S. 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 ( DIS ) 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

scrutinized is an interview with ABC News on Friday that will be

aired in full at 8 p.m. 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.

SEEN BY DOCTOR

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.