WASHINGTON: Vice-President Kamala Harris wasted no time launching her 2024 presidential campaign, seeking the support of fellow Democrats with the backing of President Joe Biden after he pulled out of the race amid concerns about his age and health.

Her campaign officials and allies made hundreds of calls on behalf of Harris on Sunday, urging delegates to the Democratic Party convention next month to join in nominating her for president in the Nov 5 election against Republican Donald Trump.

Multiple sources said the calls, aimed at blocking would-be Democratic challengers, began almost immediately after Biden, 81, abandoned the race.

At the same time, Democratic state party chairs backed Harris in a phone call, several participants said.

Harris spoke with Pennsylvania Governor Josh Shapiro, a potential vice-presidential running mate, House Democratic leader Hakeem Jeffries, and Congressional Black Caucus chairman Representative Steven Horsford, said a source familiar with the matter.

Harris, 59, who is black and Asian-American, would fashion an entirely new dynamic with Trump, 78, offering a vivid generational and cultural split-screen.

The United States has yet to elect a woman president in its 248-year history.

"Harris will be easier to beat than Joe Biden would have been," Trump said after Biden's announcement.

Biden, the oldest person ever to have occupied the Oval Office, said he would remain in the presidency until his term ends on Jan. 20, 2025, while endorsing Harris to run for president in his place.

Facing mounting questions about his mental acuity, Biden is the first sitting president to give up his party's nomination for reelection since President Lyndon B. Johnson during the Vietnam War in March 1968.

Biden's withdrawal leaves his replacement with less than four months to wage a campaign. Prominent Democrats, including potential Harris challengers such as California Governor Gavin Newsom, immediately backed the vice-president.

Harris said: "My intention is to earn and win this nomination. I will do everything in my power to unite the Democratic Party, and unite our nation, to defeat Trump."

Despite the early show of support for Harris, talk of an open convention when Democrats gather in Chicago Aug 19 to 22 was not totally silenced.

Former House speaker Nancy Pelosi and former president Barack Obama did not announce endorsements, although both praised Biden.

Two other potential challengers- Michigan Governor Gretchen Whitmer and Kentucky Governor Andy Beshear - made no mention of the vice-president in their statements.

With Democrats wading into uncharted territory, Democratic National Committee chairman Jaime Harrison said the party would soon announce the next steps in its nomination process.

What is certain is that Biden's withdrawal has again reshaped a White House contest shaken repeatedly during the last month.

On June 27, Biden's poor performance in a debate with Trump led many Democrats to urge him to drop out. Then on July 13, a gunman attempted to assassinate Trump.

And last week Trump named hardline Republican US Senator J.D. Vance, 39, to serve as his vice-presidential running mate.

ABORTION RIGHTS LEADER

A former attorney-general of California and former US senator, Harris ran unsuccessfully for the Democratic Party's presidential nomination in 2020.

Biden won the nomination, picked Harris to be his vice-president, and went on to beat Trump.

Harris has been outspoken on abortion rights, an issue that resonates with younger voters and progressives.

She is expected to stick largely to Biden's foreign policy playbook on such issues as China, Iran and Ukraine, but could strike a tougher tone with Israel over the Gaza war if she tops the Democratic ticket and wins the November election.

Proponents argue she would energise those voters, consolidate black support and bring sharp debating skills to prosecute the political case against the former president.

But some Democrats were concerned about a Harris candidacy, in part because of the weight of a long history of racial and gender discrimination in the US.

Polling shows that Harris performs no better statistically than Biden had done against Trump.

In a head-to-head match-up, Harris and Trump were tied with 44 per cent support each in a July 15 to 16 Reuters/Ipsos poll conducted after the July 13 assassination attempt on Trump.

Trump led Biden 43 per cent to 41 per cent in that same poll, though the two percentage point difference was not meaningful considering the poll's three-point margin of error.

Biden's campaign had US$95 million on hand at the end of June, according to a filing with the Federal Election Commission. Campaign finance law experts disagree on how easily that money could be shifted to a Harris-led campaign.

Small-dollar donors raised more than US$46.7 million on ActBlue in the first five hours of Harris' presidential campaign, the fundraising platform said on X on Sunday.

LAST-MINUTE SHIFT

Biden, who said he would address the nation this week, has not been seen in public since testing positive for Covid-19 last week. He was isolating at his home in Rehoboth Beach, Delaware.

"While it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as president for the remainder of my term," Biden wrote on X.

Congressional Republicans argued on Sunday that Biden should resign as president immediately, which would turn the White House over to Harris and put House of Representatives Speaker Mike Johnson, a Republican, next in line in succession.

"If he's incapable of running for president, how is he capable of governing right now? I mean, there are five months left in this administration. It's a real concern, and it's a danger to the country," Johnson said.

  - Reuters