Washington DC - Democratic Party donors have poured a record amount into Vice President Kamala Harris's campaign in the 24 hours since Joe Biden stepped down from the White House race and endorsed his erstwhile running mate.

"Team Harris raised $81 million in its first 24 hours, adding to the existing near quarter-of-a-billion dollar war chest already amassed this election cycle," her campaign said in a statement Monday.

It claimed the haul was the "largest 24-hour raise in presidential history."

Harris (59) swiftly secured the backing of a flood of Democrats after receiving Biden's endorsement on Sunday – including from California Governor Gavin Newsom and Pennsylvania Governor Josh Shapiro, both touted as top presidential contenders.

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She still needs to persuade key hold-outs and convince some donors who are urging for an open contest to select the Democratic candidate that will take on Republican challenger Donald Trump.

But for now, her campaign appears to have energized large and small donors alike – a turnaround from the uncertain period after Biden's disastrous debate performance against Trump, when major donors reportedly halted fundraising.

On Monday, US media reported that Democratic super political action committee (PAC) Future Forward had locked in $150 million in donor commitments within 24 hours of Biden dropping out of the race, citing a senior aide.

The super PAC held some $122 million in cash as of the end of June, according to Federal Election Commission filings.

Among the major names to back Harris on Sunday was LinkedIn co-founder Reid Hoffman.

He wrote on social media: "I wholeheartedly support Kamala Harris and her candidacy for President of the United States in our fight for democracy in November."

Open Society Foundations chairman Alex Soros, son of billionaire philanthropist George Soros, also called for the public to "unite around Kamala Harris and beat Donald Trump."

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The Soros family are known to be mega-donors, spending tens of millions per election cycle.

Evercore founder Roger Altman told CNBC on Monday that as a Wall Street donor, he would line up behind Harris as well – adding that he expects her campaign to be "very well-financed."

"Suddenly, the Democratic base has gone from demoralized to excited," he said.

Swing Left, a grassroots group that launched a fund for the eventual Democratic nominee on Sunday, told AFP it raised over $160,000 in less than 24 hours from some 1,500 unique donors.

But others have urged an open selection process.

Donor Vinod Khosla called for an "open convention" and "a more moderate candidate" in a social media post shortly after Biden said he would not run for reelection.

Another donor, John Morgan, wrote: "You have to be enthusiastic or hoping for a political appointment to be asking friends for money. I am neither."