US President Joe Biden told congressional Democrats on Monday that he is "firmly committed" to running for re-election against his Republican rival Donald Trump and called for speculation over his candidacy to end despite mounting concerns about his mental fitness and the viability of his campaign.

In a strongly-worded letter to some donors and Democrats who are demanding that he quit the race for the White House on November 5, Biden, the presumptive candidate of the Democratic Party, sought to address their growing concerns.

"I have heard the concerns that people have — their good faith fears and worries about what is at stake in this selection. I am not blind to them,” 81-year-old Biden wrote in the letter to the Democratic lawmakers in the US Congress.

“I can respond to all this by saying clearly and unequivocally: I wouldn’t be running again if I did not absolutely believe I was the best person to beat Donald Trump in 2024,” asserted the president, who after his June 27 disastrous debate performance has been receiving calls to drop out of the race.

Biden has insisted that he is in the race and he will not give up.

So far only a small section of the Democratic leadership have asked him to leave the race to the presidency.

A majority of the party leadership has been supporting him so far. He has conceded that it was a bad night, but insists that one 90-minute debate cannot wash away his three-and-half years of performance that revived the economy, reduced unemployment and restored America’s position in the world.

The incumbent president said that millions of Democratic party members had voted in his favour during the primaries and asked the Democratic lawmakers if they would suggest that the process did not matter by urging him to step aside.

“I decline to do that. I feel a deep obligation to the faith and the trust the voters of the Democratic Party have placed in me to run this year. It was their decision to make. Not the press, not the pundits, not the big donors, not any selected group of individuals, no matter how well-intentioned,” he said.

“The voters – and the voters alone – decide the nominee of the Democratic Party. How can we stand for democracy in our nation if we ignore it in our own party? I cannot do that. I will not do that,” he said.

“The question of how to move forward has been well-aired for over a week now. And it’s time for it to end. We have one job. And that is to beat Donald Trump. Any weakening of resolve or lack of clarity about the task ahead only helps Trump and hurts us,” Biden wrote.

"I want you to know that despite all the speculation in the press and elsewhere, I am firmly committed to staying in this race, to running this race to the end, and to beating Donald Trump,” Biden wrote in the letter.

On Sunday, at least five Democratic lawmakers expressed their view on Sunday that President Joe Biden should step down from the US presidential election scheduled for November 5, according to multiple news reports.

The lawmakers -- Jerry Nadler, Mark Takano, Joe Morelle, Ted Lieu and Adam Smith -- expressed their view in this regard during a Democratic Party phone call organised to discuss Biden's disastrous debate performance against Trump, in Atlanta on June 27.

Biden's approval ratings have come down and his own party colleagues have started questioning his health and ability to govern the country for the next four years. Biden has asserted that he is staying in the race and exuded confidence that he will win the election against Trump in November.

House Minority Leader Hakeem Jeffries had convened a virtual meeting of his party colleagues in the House of Representatives to discuss the evolving political scenarios after the June 27 presidential debate between Biden and Trump.

The virtual meeting was billed as a "listening session" to get inputs from members about the viability of Biden's candidacy, "The New York Times" reported, adding that it was clear before the call that many top Democrats felt strongly that Biden had to go.

Congressman Smith, the ranking member of the Armed Services Committee, said it is time for Biden to go, according to two people familiar with the call, the daily reported. The four other Congressmen also felt the same and believed it was time for Biden to exit the race.

"Lieu was the highest-ranking person in the House Democratic leadership to support pressing for Biden to not seek reelection," "The Washington Post" reported.

"The developments opened a wide crack in Biden's wall of support, bringing the number of House Democrats publicly or privately saying he should step aside to about 10. They included the top Democrats on the Judiciary, Armed Services and Intelligence committees. The defections cast further doubts on Biden's path forward, even as he largely controls his own fate thanks to sweeping the Democratic presidential primaries," "The Wall Street Journal" reported.

Meanwhile, Biden was in Pennsylvania on Sunday, addressing rallies and meeting people in the key battleground state.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.