US Vice President Kamala Harris has assailed Donald Trump at her first campaign rally since replacing President Joe Biden as the Democratic presidential candidate.

In a 17-minute speech on Tuesday (US time), Harris aggressively went after Trump, comparing her background as a former prosecutor to his record as a convicted felon.

“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. So hear me when I say: I know Donald Trump’s type,” she said.

“I promise you, I will proudly put my record against his any day of the week.”

Harris ticked through a list of priorities, saying that if elected she would expand abortion access, make it easier for workers to join unions and address gun violence, drawing a sharp contrast with Trump, the Republican nominee for president in the November 5 election.

She also took aim at right-wing Republicans’ “extreme Project 2025 agenda”.

“Can you believe they put that thing in writing? Read it. It’s 900 pages,” she said, arguing that Project 2025 would cut social security and Medicare and give tax breaks to the wealthy.

The raucous rally at West Allis Central High School in a Milwaukee suburb was a notable contrast to the smaller, more subdued events held by Biden.

The audience of several thousand danced and waved Harris signs, while chants of “Ka-ma-la!” broke out when she took the stage. The reception underscored Democrats’ hopes that 59-year-old Harris can revive what had been a flagging campaign under Biden, 81.

“Donald Trump wants to take our country backward,” she said.

“Do we want to live in a country of freedom, compassion and rule of law, or a country of chaos, fear and hate?”

Harris emphasised her commitment to reproductive rights, an issue that has plagued Republicans since the US Supreme Court – powered by three Trump-appointed justices – eliminated a nationwide right to abortion in 2022.

It came as the first poll since Biden dropped out of the election race and endorsed Harris as his successor showed she was in front of Trump 44 per cent to 42 per cent among registered voters.

The national Reuters/Ipsos poll was conducted on Monday and Tuesday.

A similar poll last week – with Biden still in the race – showed him trailing Trump by two percentage points.

Both were within the poll’s three-point margin of error. But the results could signal movement in Democrats’ direction – and may suggest that Harris’ elevation to the top of the ticket blunted whatever momentum Trump hoped to gain from last week’s Republican National Convention.

In another development on Tuesday (local time), the Trump campaign has lodged a formal complaint about Harris’s takeover of millions of dollars donated to Biden before he quit the presidential race.

“Kamala Harris is seeking to perpetrate a $91.5 million ($138 million) heist of Joe Biden’s leftover campaign cash – a brazen money grab that would constitute the single largest excessive contribution and biggest violation in the history of the Federal Election Campaign Act of 1971, as amended,” campaign lawyer David Warrington wrote to the US Federal Election Commission, according to a report in the New York Times.

Harris’s campaign has broken fundraising records this week, raking in more than $120 million in its first 24 hours.

Warrington’s complaint demands a criminal investigation – and that Biden be ordered to refund donations instead of transferring them to Harris.

Trump hits back

Trump and his allies have tried to tether Harris to some of Biden’s more unpopular policies, including his administration’s handling of the surge of migrants at the southern border with Mexico.

In a conference call on Tuesday, Trump expressed confidence in his ability to defeat Harris, noting that her previous presidential run in 2020 did not survive even until the first statewide nominating contest.

Trump offered to debate Harris multiple times. Trump and Biden had one more debate scheduled for September 10 after their encounter on June 27. Biden’s poor performance that night sparked Democratic calls for him to step aside.

“I want to debate her, and she’ll be no different because they have the same policies,” Trump said.

Harris swiftly consolidated her party’s support after Biden abandoned his re-election campaign under pressure from members of his party who worried about his ability to beat Trump, 78, or to serve for another four-year term.

She wrapped up the nomination on Monday night by winning pledges from a majority of the delegates who will determine the nominee at next month’s party convention, the campaign said.

Most Democratic lawmakers have lined up behind her candidacy. They include party leaders in the Senate and House, Chuck Schumer and Hakeem Jeffries, who endorsed Harris on Tuesday at a joint press conference.

Biden said on X that he would deliver a Wednesday night Oval Office speech explaining his decision to end his campaign. He returned to Washington on Tuesday after spending several days isolating at home with Covid-19.

Biden’s dramatic exit followed Trump’s narrow survival of a July 13 assassination attempt that raised questions about security failures in the US Secret Service. The agency director, Kimberly Cheatle, resigned on Tuesday.

-with AAP