WASHINGTON -- In a letter to Democratic lawmakers on Monday, US President Joe Biden, the presumptive Democratic presidential nominee, vowed to stay in the race despite concerns about his physical and mental fitness following his poor performance in a debate with former President Donald Trump.

"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.

"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," Biden said.

Biden noted that there are only 42 days to the Democratic Convention, when the party's presidential nominee will be formally announced, and 119 days to the general election.

"Any weakening of resolve or lack of clarity about the task ahead only helps Trump and hurts us," Biden concluded. "It is time to come together, move forward as a unified party, and defeat Donald Trump."

Biden's appeal for unity followed calls from some Democratic lawmakers asking him to exit the race, prompted by his significant struggle in a debate against Trump on June 27.

According to the average polling data from Real Clear Politics, Trump leads Biden by 3.3 percentage points in the general election.