Biden had planned to raise money in Austin, Denver and California next week, but these plans have been shelved, multiple Democratic sources have told Reuters.
The president has tested positive for COVID-19 but said on Friday he would return to the campaign trail next week.
However Biden's campaign have denied the claims, saying his fundraisers would go ahead as planned.
Former late-night comic David Letterman is to headline a fundraiser event for Biden at the home of Hawaii Governor Josh Green in about 10 days, a sign of forward movement for Biden.
Comedian David Letterman is to headline a campaign fundraiser for Joe Biden. (AP PHOTO)
But the sources say a number of big donors are closing their chequebooks, using their financial clout to tell Biden to drop out of the November 5 race, potentially in favour of Vice President Kamala Harris.
The campaign hoped to raise some $50 million in big-dollar donations in July for the Biden Victory fund but was on track for less than half that figure as of Friday, according to two sources familiar with the fundraising efforts.
"There are a lot of donors who have said they won't put another dime in this race. The question is if Biden stays in the race, will they come back?" said one major East Coast campaign financier.
Biden raised $28 million in one night in June at a Hollywood fundraiser that was hosted by the actor George Clooney, who later urged Biden to end his campaign.
US Vice President Kamala Harris has been more visible since Joe Biden tested positive for COVID-19. (EPA PHOTO)
Harris was to speak to major donors on Friday at the request of advisers to the president, according to a source familiar with the plan.
Fundraising from megadonors was expected to drop from June to July due to many of these people taking vacations, a campaign official told Reuters on Friday. "This narrative that high-dollar fundraising has dried up is wrong," the official said.
The official noted that the campaign has 10 campaign fundraisers on the schedule for this month.
Some 34 congressional Democrats have now publicly called on the incumbent to drop out following a disastrous June debate against Republican Donald Trump that raised questions about Biden's ability to win or to carry out his duties for another four years.
Meanwhile the official party process to nominate Biden is advancing.
The Democratic National Convention Rules Committee met on Friday to outline a virtual voting process to bring forward the official nomination of the 81-year-old before the party's in-person convention starts on August 19 in Chicago.
Officials argued an early nomination is needed to more easily comply with state election laws or risk being left off the November ballot.
For a party already divided over Biden, the early virtual vote is another point of controversy.
A member of the rules committee asked whether it was possible that another candidate could challenge Biden in a virtual vote. Leah Daughtry, the committee's co-chair, said any challenger would need the "verified support of hundreds of delegates."
"Such a challenge has never happened over the past half century of competitive primaries," Daughtry said.
It was unclear how the nominating process would unfold if Biden were to abandon his re-election bid. The committee is expected to meet again next week to finalise the plan.
Biden has been isolating since he tested positive for COVID this week and was believed to be taking calls to step aside seriously.
Biden has insisted for weeks that he would stay in the race despite calls from heavyweights in his party to cede his position.
Some Democrats have begun advertising against Biden. Pass the Torch, a group that wants Biden to step down, has launched a TV ad to air in Washington and Rehoboth, Delaware, where Biden frequently vacations, and features Democratic voters from Pennsylvania urging Biden to "pass the torch."
