US Vice President Kamala Harris, building Democratic backing for her sudden presidential run, rallied supporters on Monday (Jul 22) with a debut campaign speech vowing to go after Republican nominee Donald Trump like the courtroom prosecutor she once was.

"I took on perpetrators of all kinds. Predators who abused women, fraudsters who ripped off consumers, cheaters who broke the rules for their own gain," Harris told campaign workers 28 hours after President Joe Biden, 81, abandoned the 2024 White House race and endorsed her.

"So hear me when I say I know Donald Trump's type. In this campaign, I will proudly, I will proudly put my record against his," said Harris, who was attorney general of California and a US senator before serving as Biden's vice president.

The Trump campaign responded to Harris' comments.

"Kamala Harris is just as incompetent as Joe Biden and even more liberal," said Karoline Leavitt, the campaign's national press secretary.

"Not only does Kamala need to defend her support of Joe Biden’s failed agenda over the past four years, she also needs to answer for her own terrible weak-on-crime record in California."

Trump is due to be sentenced in September after having been found guilty of falsifying business records to hide hush money payments to a porn star. He also faces criminal charges related to his efforts to overturn Biden's 2020 victory. He falsely claims he lost because of election fraud.

Biden, who pulled out of the race on Sunday amid questions about his age and health, called into Harris' campaign event. Recovering from COVID-19 at his home in Delaware, he sounded hoarse but appreciative of his vice president.

He said he thought he had made the right decision by dropping out. Biden, the oldest person to occupy the Oval Office, said on Sunday he would remain in the presidency until his term ends on Jan 20, 2025.

Harris, 59, outlined a series of policies she promised to pursue including signing laws to protect abortion rights and ban assault rifles and making rebuilding the middle class the focus of her presidency.

Within minutes of receiving Biden's backing on Sunday, Harris began consolidating Democratic support for her presidential bid, securing commitments from hundreds of convention delegates, announcing a massive fundraising haul and earning endorsements from top party figures.

These included former Speaker Nancy Pelosi, who has remained influential since stepping down as the party's House of Representatives leader in 2022. The AFL-CIO labour union federation, which represents 12.5 million workers, said on Monday it had also endorsed Harris for president.

The Harris campaign aims to secure commitments from a majority of the nearly 4,000 Democratic delegates to next month's Democratic Party convention by Wednesday evening, sources told Reuters, effectively wrapping up the nomination.

Campaign officials and allies have made hundreds of calls urging delegates to nominate Harris for president in the Nov 5 election.

Harris' campaign said it raised US$81 million in the 24 hours following Biden's exit, the most for a single day in the 2024 campaign for either party.

Virtually all of the prominent Democrats who had been seen as potential challengers to Harris have declared support for her, including Governors Gretchen Whitmer of Michigan, Gavin Newsom of California and Andy Beshear of Kentucky.

Biden's departure was the latest shock to a White House race that included his disastrous Jun 27 debate performance against former President Trump and the Jul 13 near-assassination of Trump by a gunman during a campaign stop.

Harris lauded Biden for his service to the country. At a White House event to honour college athletes earlier, she said: "Joe Biden's legacy over the last three years is unmatched in modern history."

Harris will travel to Milwaukee on Tuesday in the battleground state of Wisconsin which last week hosted a Republican National Convention that offered a stark display of Trump's dominance over his party.