US President Joe Biden on Monday again rebuked calls from fellow members of his Democratic Party to call off his reelection campaign.

Some Democrats in Congress have urged Biden to end his reelection bid over concerns that he could lose to his rival, former Republican President Donald Trump, concerns which were only exacerbated by a poor showing in the recent TV debate.

Later on Monday, White House spokesperson Karine Jean-Pierre said that Biden was not being treated for Parkinson's and had seen a neurologist three times.

Biden, 81, insisted in a letter to congressional Democrats returning to Washington after the Independence Day recess that he remains "firmly committed" to the campaign.

"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.

"The question of how to move forward has been well-aired for over a week now, and it's time for it to end," he continued.

"We have 42 days to the Democratic Convention 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. It's time to come together, move forward as a unified party, and defeat Donald Trump."

Biden: 'I am not going anywhere'

Following up the letter with a phone-in on MSNBC's "Morning Joe" program, Biden insisted that "average democrats" are still behind him and insinuated that calls for him to step aside are only eliminating from "elites" and senior party officials.

"They're big names, but I don't care what those big names think," he said, daring potential opponents to "announce for president" and "challenge me at the convention" or else support him. "I am not going anywhere," he stated.

He called Trump "the most extreme candidate" he has ever come across and, referring to Sunday's French election, said "France rejected extremism and Americans will, too."

Biden's wife, First Lady Jill Biden declared on Monday she is "all in" on her husband's bid to remain as the presumptive Democratic presidential nominee, AP news agency reported.

Amid all the speculation about the President's worsening health, Biden's doctor has not seen reason to re-evaluate him for Parkinson's disease, the White House said. 

Who could replace Joe Biden?

Alternative candidates suggested by Democrats to replace Biden as the party's presidential candidate would include current Vice President Kamala Harris, who would only trail Trump by two percentage points according to a recent CNN poll, and the Governor of California Gavin Newsom, who would trail Trump by five points. Biden is currently polling at around six points behind.

Michigan Governor Gretchen Whitmer is another potential candidate, although she is reported to be personally opposed to the idea of replacing Biden.

Indeed, Democrats are split on whether to call on Biden to abandon his bid or to redouble their efforts behind the veteran politician, still considered the best candidate to unite all wings of the party.

In an effort to get everyone "on the same page," House Democratic leader Hakeem Jeffries is set to convene lawmakers for private meetings on Monday before he reveals his own preference, according to the AP news agency.

wd,mf/fb (Reuters, AP)