Biden and top aides on Friday vowed to continue with the campaign, even as major donors signalled they were unwilling to open their chequebooks unless the 81-year-old president stepped aside.
The crisis of confidence in Biden's ability to win has placed a huge spotlight on Harris, widely believed to be the most likely replacement if he steps down.
Her fundraising events, including the one on Saturday in Provincetown, Massachusetts are getting added interest from donors who want to signal they are willing to coalesce around her potential bid for the White House, according to three Democratic fundraisers.
The fundraiser is being hosted by celebrity wedding planner Bryan Rafanelli, with Transportation Secretary Pete Buttigieg expected to speak, according to a person familiar with the event.
Thirty-five congressional Democrats, representing more than 10 per cent of the party's members in Congress, have now publicly called on Biden, who is isolating at his Delaware home with a case of COVID-19, to drop out.
This followed Biden's disastrous debate last month against Republican former President Donald Trump that raised questions about the incumbent's ability to win the November 5 election or carry out his duties for another four years.
Democratic former President Bill Clinton and former US Secretary of State Hillary Clinton have privately been supportive of Biden's decision to stay in the race and have been actively encouraging donors to stay with him, NBC News reported on Saturday, citing two sources familiar with the matter.
Separately, US Representative Mark Takano of California on Saturday became the latest Democratic congressman to call on Biden to step aside as a presidential candidate.
Biden's campaign hoped to raise some $US50 million ($A75 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.
The campaign called reports of a July fundraising slump overstated, noting that it anticipated a drop-off in large donations due to vacations. It said the campaign still has 10 fundraisers on the schedule this month.
Harris assured major Democratic donors on Friday that the party would prevail in the presidential election as more lawmakers called for her running mate, Biden, to stand down.
"We are going to win this election," she said on a call arranged on short notice to calm donors, according to a person on the call.
"We know which candidate in this election puts the American people first: Our president, Joe Biden."
Harris attended the call "at the direct request of senior advisers to the president," one of the people said, an account confirmed by another person familiar with the matter.
