United States President Joe Biden has slapped down calls to end his faltering re-election bid, insisting he is “not going anywhere”.

Biden, 81, has been under growing pressure to assuage doubts about his physical and mental fitness following a stumbling debate performance last week against his Republican challenger, Donald Trump.

Three elected Democrats have publicly called on Biden to step aside since Thursday’s debate, during which the president stumbled over his words and lost his train of thought on several occasions.

Several other Democratic lawmakers have publicly warned that they expect him to lose to Trump in November’s presidential election.

Addressing a supporter who yelled “keep up the fight!” during an Independence Day celebration on Thursday, Biden reiterated that he had no intention of quitting the race.

“You got me, man. I’m not going anywhere,” Biden said at the White House event.

In a pre-recorded radio interview that aired earlier on Thursday, Biden pledged to keep fighting, describing his faltering debate performance as “a bad night”.

“I screwed up. I made a mistake,” Biden said. “That’s 90 minutes on stage. Look at what I’ve done in 3.5 years.”

Despite growing concerns about Biden’s electability among Democrats and donors, the president has so far avoided a mass revolt against his candidacy from within his own ranks.

On Wednesday, a group of Democratic governors reiterated their support for Biden after a meeting with him and Vice President Kamala Harris at the White House.

“The president has always had our backs. We’re going to have his back as well,” Maryland Governor Wes Moore told reporters.

Still, cracks continued to emerge in Biden’s support base.

On Thursday, Seth Moulton, a congressman from Massachusetts, became the third elected Democrat to call on Biden to withdraw from the race.

“President Biden has done enormous service to our country, but now is the time for him to follow in one of our founding fathers George Washington’s footsteps and step aside to let new leaders rise up and run against Donald Trump,” Moulton told Boston radio station WBUR.

Abigail Disney, the heiress to the Disney family fortune, said she would no longer donate to the Democrats as long as Biden stayed in the race.

“This is realism, not disrespect,” Disney told CNBC. “Biden is a good man and has served his country admirably, but the stakes are far too high.”

The United Kingdom’s Economist also added its voice to a growing list of media outlets, including The New York Times and Boston Globe, that have urged the president to step aside.

Concerns about Biden’s age and mental acuity, a longstanding concern for voters, appear to have intensified among the public since his debate appearance.

In a New York Times/Siena College poll released on Wednesday, nearly three-quarters of registered voters said Biden was too old to do a second term.

Biden also trailed Trump 49 percent to 41 percent, the highest margin since 2015.

If Biden were to quit, it would cast the race into uncharted territory.

The US presidential primary season, when party members typically vote for their preferred candidate, has already ended, although the party’s candidate will not be finalised until the Democratic National Convention next month.

Since Biden won nearly all of the party’s delegates, it is unlikely that he could be forced to step aside against his will.

Vice President Kamala Harris, who has rallied behind Biden, is widely considered the most likely successor if Biden were to step aside – although she, like other potential nominees, also trails Trump in polling.

California Governor Gavin Newsom, Michigan Governor Gretchen Whitmer and Pennsylvania Governor Josh Shapiro have also been floated as possible replacements.