Washington DC - President Joe Biden on Monday restated his determination to stay in the presidential race despite increasingly widespread calls for him to drop out and urged Democratic lawmakers to show full support for his re-election campaign

"I am firmly committed to staying in the race," Biden wrote in a letter to his fellow Democrats, a growing number of whom are increasingly skeptical of him.

Seeking to put his disastrous performance in the first presidential debate behind him, the 81-year-old acknowledged "good faith fears and worries" that he wouldn't be able to beat Donald Trump in November, but continued to insist he was "the best person" for the job.

Biden then pointed out he had easily won the Democratic primaries, which featured very little competition, but was being beset by protest votes over his support for Israel's brutal war on Gaza.

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"Only three people challenged me," he wrote, before adding: "One fared so badly that he left the primaries to run as an independent" – an apparent swipe at Independent presidential candidate Robert F. Kennedy, who in October quit his push to secure the Democratic nomination.

Biden benefited from a revamped primary schedule that worked in his favor, as well as being the only name on the ballot in states such as Florida.

The rest of his letter was dedicated to highlighting the dangers of a second Trump presidency, the possibility of which would increase if there were any "weakening of resolve or lack of clarity."

"It is time to come together, move forward as a unified party, and defeat Donald Trump," he wrote.

Biden also called into to MSNBC's Morning Joe, which is rumored to be a staple of the president's TV diet.

"I'm not going anywhere," he told hosts Joe Scarborough and Mika Brzezinski, before blaming his debate performance on a "bad night" and party elites for driving the narrative that he should back out.

"Any of these guys that don’t think I should run, run against me," Biden proposed. "Announce for president, challenge me at the convention."

So far, five Democratic lawmakers have publicly called on Biden to quit the race and make way for a better candidate. Polls show a majority of voters think he is too old to be president, while just over 50% of Democrats think he should continue his campaign.