Vice President Kamala Harris has moved swiftly to attract Democratic delegates to her campaign for the White House after President Joe Biden stepped aside amid concerns within the party that he would be unable to defeat Donald Trump.

Mr Biden’s exit on Sunday, prompted by Democratic worries over his fitness for office, was a seismic shift to the presidential contest that upended both parties’ plans for the race.

Aiming to put weeks of drama over Mr Biden’s candidacy behind them, prominent Democratic elected officials, party leaders and political organisations quickly lined up behind Ms Harris in the hours after the president announced he was dropping his re-election campaign.

Ms Harris, who Mr Biden backed after ending his candidacy, is the only declared candidate so far and was working to quickly secure endorsements from a majority of delegates.

It is the first item on a big political to-do list for her after Mr Biden’s decision to exit the race, which she learned about on a Sunday morning call with the president.

If she wins the nomination, she must also pick a running mate and shift a massive political operation to boost her candidacy with just over 100 days until election day.

On Sunday afternoon, Mr Biden’s campaign formally changed its name to “Harris for President”, reflecting that she is inheriting his political operation of more than 1,000 staff and a war chest that stood at nearly 96 million dollars (£74 million) at the end of June.

On Monday morning, campaign spokeswoman Lauren Hitt said Ms Harris had raised 49.6 million dollars (£38.3 million) in donations in the 15 hours after Mr Biden’s endorsement.

Ms Harris spent much of Sunday surrounded by family and staff, making more than 100 calls to Democratic officials to line up their support for her candidacy, according to a source.

It comes as she tries to move her party past the painful public wrangling that had defined the weeks since Mr Biden’s disastrous June 27 debate with Mr Trump.

Donald Trump raises a clenched fist in front of supporters
Donald Trump at a campaign rally on Saturday (Evan Vucci/AP)

Speaking to party leaders, Ms Harris expressed gratitude for Mr Biden’s endorsement, but insisted she was looking to earn the nomination in her own right, the source said.

She quickly won endorsements from the leadership of several influential caucuses and political organisations, including the AAPI Victory Fund, which focuses on Asian American and Pacific Islander voters, the Collective PAC, focused on building black political power, and the Latino Victory Fund, as well as the heads of the Congressional Progressive Caucus, the Congressional Hispanic Caucus and the entire Congressional Black Caucus.

Ms Harris, if elected, would be the first woman and first person of south Asian descent to be president.

A handful of men who had already been discussed as potential running mates for Ms Harris — Pennsylvania governor Josh Shapiro, North Carolina governor Roy Cooper and Arizona senator Mark Kelly — swiftly issued statements endorsing her.

Former president Barack Obama held off on an immediate endorsement, as some in the party have expressed worry that the quick shift to Ms Harris would appear to be a coronation, instead pledging his support behind the eventual party nominee.

Joe Biden raises the hand of Kamala Harris at the White House
Joe Biden has endorsed Kamala Harris’s candidacy (Evan Vucci/AP)

West Virginia senator Joe Manchin, who left the party earlier this year but considered re-registering as a Democrat to compete for the nomination against the vice president, told CBS News on Monday that he would not be a candidate.

Additional endorsements on Monday, including Maryland governor Wes Moore and Kentucky governor Andy Beshear, left a dwindling list of potential rivals to Ms Harris, who is still the only candidate.

Ms Harris is to make a public appearance on Monday morning at the White House, where she is scheduled to speak at an event honouring National Collegiate Athletic Association championship teams. She is filling in for Mr Biden, who is recovering after contracting Covid-19 last week.

Ms Harris, in a statement, praised the president’s “selfless and patriotic act” in deciding to leave the race and said she intends to “earn and win” her party’s nomination.

“I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump and his extreme Project 2025 agenda,” she said.

Mr Biden plans to discuss his decision to step aside later this week in an address to the nation. He wrote in a letter posted on Sunday: “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.”

Nearly 30 minutes after he delivered the news that he was ending his campaign, he threw his support behind Ms Harris.

“Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year,” he said in another post. “Democrats — it’s time to come together and beat Trump.”