US Vice President Kamala Harris has gained the crucial endorsement of former House Speaker Nancy Pelosi as she made her first public appearance since Joe Biden dropped his presidential campaign.

Pelosi, who was instrumental in urging Biden to step aside, broke her silence to officially support Harris as the Democrat nominee.

“Today it is with immense pride and limitless optimism for our country’s future, that I endorse Vice President Kamala Harris for President of the United States,” she said.

“My enthusiastic support for Kamala Harris for president is official, personal and political.

“Politically, make no mistake: Kamala Harris as a woman in politics is brilliantly astute – and I have full confidence that she will lead us to victory in November.”

Pelosi had not previously revealed whether she would back Harris or call for an open nomination process.

Virtually all of the prominent Democrats who had been seen as potential challengers to Harris have lined up behind her, including governors Gretchen Whitmer of Michigan, Gavin Newsom of California and Andy Beshear of Kentucky.

Democratic Pennsylvania senator John Fetterman – one of Biden’s most vocal supporters in recent weeks – also endorsed Harris, saying he was “all in for the next president”.

At her first public appearance since Biden abruptly pulled out of the election race, Harris lauded him for his service to the US and said his legacy of the past three years was “unmatched in modern history”.

“Every day, our President, Joe Biden, fights for the American people and we are deeply, deeply grateful for his service to our nation,” she said at a White House event to honour college athletes.

Harris did not specifically refer to her status as the leading Democratic candidate for president, after Biden stepped aside on Sunday (US time) under growing pressure from fellow Democrats.

Meanwhile, Republican candidate Donald Trump exploded on social media about his new opponent and old foe.

“Joe Biden will go down as the Worst President in the history of the United States,” he posted on Truth Social.

“KAMALA, OUR HORRIBLE & INCOMPETENT BORDER CZAR, WILL BE WORSE!”

Democrat campaign officials and allies have already made hundreds of calls on Harris’ behalf, urging delegates to next month’s Democratic Party convention to join in nominating her for president in the November 5 election against Trump.

Biden’s departure was the latest shock to a White House race that included the near-assassination of former president Trump by a gunman during a campaign stop and the nomination of US Senator JD Vance as Trump’s running mate.

Harris said in a post on X that she would visit what had been the Biden campaign headquarters in Delaware – now the Harris campaign headquarters – on Monday (US time).

She said Biden, who tested positive for Covid-19 last week, was feeling better.

“My intention is to earn and win this nomination,” Harris said on Sunday.

“I will do everything in my power to unite the Democratic Party – and unite our nation – to defeat Donald Trump.”

Biden, at 81 the oldest person to have occupied the Oval Office, said he would remain Preisdent until his term ends on January 20, 2025.

He has not been seen in public since testing positive for Covid last week.

Biden is isolating at his home in Rehoboth Beach, Delaware, and tentatively plans to meet Israeli Prime Minister Benjamin Netanyahu on Tuesday if he has recovered.

His shaky June 27 debate performance against Trump led senior Democrats to urge him to end his run. But senior Republicans have demanded he also resign from office, arguing that if he is not fit to campaign, he is not fit to govern.

Harris spent Sunday working the phones, dressed in a Howard University sweatshirt and eating pizza with anchovies as she 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, according to sources.

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

Former president Barack Obama has not announced his endorsement.

In a post on X that did not mention Harris, billionaire Michael Bloomberg, a major Democratic donor, encouraged the party to “take the pulse of voters” before making a decision.

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.

-with AAP