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 the 81-year-old Biden 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 chair Representative Steven Horsford, said a source familiar with the matter.

Harris, a 59-year-old woman 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 asserted on CNN shortly 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.

The White House said Harris would deliver remarks on the South Lawn at 11:30 a.m. on Monday at an event celebrating the NCAA 2023-24 college championship teams.

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.

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

Despite the early show of support for Harris, talk of an open convention when Democrats gather in Chicago Aug. 19-22 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 Chair 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 former President Trump.

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

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 energize 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 United States.

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 percent support each in a July 15-16 Reuters/Ipsos poll conducted immediately after the July 13 assassination attempt on Trump.

Trump led Biden 43 percent to 41 percent in that same poll, though the 2 percentage point difference was not meaningful considering the poll's 3-point margin of error.

Biden's campaign had $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 $46.7 million on ActBlue in the first five hours of Harris' presidential campaign, the fundraising platform said on X on Sunday.