However, any move to bypass Harris would open up ugly divisions in party ranks, four months from election day and less than a month from the party’s national convention in Chicago. By Sunday night (Washington time), she was the only declared candidate and was working to quickly secure endorsements from a majority of Democrat delegates.

It’s only the first item on a staggering political to-do list for Harris after Biden’s decision, which she learned of on a Sunday morning call with the president. If she’s successful at locking up the nomination, she must also pick a running mate and pivot a massive political operation to boost her candidacy instead of Biden’s with just over 100 days until the election.

In endorsing her run, Biden said: “My very first decision as the party nominee in 2020 was to pick Kamala Harris as my vice president. And it’s been the best decision I’ve made. Today, I want to offer my full support and endorsement for Kamala to be the nominee of our party this year.

“Democrats – it’s time to come together and beat [Republican candidate Donald] Trump. Let’s do this.”

Harris later put out a statement saying she was honoured to have the president’s endorsement while noting that “my intention is to earn and win this nomination”.

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“Over the past year I have travelled across the country, talking with Americans about the clear choice in this momentous election. And that is what I will continue to do in the days and weeks ahead,” Harris said, adding that she would do everything in her power to unite her party and the nation.

“We have 107 days until election day. Together, we will fight. And together, we will win.”

Harris said Biden’s decision to pull out of the White House race was a “selfless and patriotic act”.

The timing of the announcement – coming at 1.46 on a Sunday afternoon in Washington (3.36am Monday AEST) – took the world by surprise, particularly as Biden had appeared determined to dig in and had said on Saturday that he would return to the campaign trail this week.

His defiance came despite almost 40 congressional Democrats calling for him to stand down since his disastrous debate with Trump, and stalwarts such as former House speaker Nancy Pelosi privately warning him that he could not only lose the White House but also control of the US Congress unless he withdrew.

Of particular concern was the fact that donations were starting to slow amid growing concerns about Biden’s mental acuity, which would have made it even harder for the party to campaign against Trump and his chosen running mate, J.D. Vance.

Biden’s campaign formally changed its name to Harris for President, reflecting that she is inheriting his political operation of more than 1000 staffers and a war chest that stood at nearly $US96 million ($144 million) at the end of June.

Small-dollar donors raised more than $US46.7 million on ActBlue in the first five hours of Harris’ presidential campaign, the fundraising platform said.

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Trump wrote on his Truth Social platform: “Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve – And never was!

“We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!”

Biden secured enough delegates in mid-March, clinching the Democratic nomination without any serious challengers, largely because he was the incumbent – and despite persistent concerns about his age.

At 81, Biden is already America’s oldest president and would have been 86 by the end of a second term. However, a nightmare performance in the first presidential debate in late June led to growing defections in his ranks, with the Democratic caucus splitting on whether he should step aside.

Trump, 78, has returned to the campaign trail this weekend with a party galvanised by his near-death experience and the Democrats in disarray over their leadership.

Biden embarked on a number of campaign rallies and conducted a prime-time television interview in which he fought off suggestions that he was too old for the job. But a rare, high-stakes, solo press conference at the conclusion of the NATO summit in Washington, D.C., did little to convince his doubters he still had the mental and physical ability to win the election.

While it was not a repeat of the train wreck of a performance from his first debate, Biden gave a series of shaky answers. Among several gaffes, he referred to Harris as “Vice President Trump” during the press conference, which lasted more than an hour.

In a hypothetical head-to-head match-up, Harris and Trump were tied with 44 per cent support each in a July 15-16 Reuters/Ipsos poll, conducted immediately after the assassination attempt against Trump.

Trump led Biden 43 per cent to 41 per cent in that same poll, though the 2 percentage point difference was within the poll’s 3 percentage point margin of error.