US Vice-President Kamala Harris has gone on the offensive against Donald Trump in the first rally of her White House campaign. Photo: BBC News

Speaking to a crowd of about 3,000 in the battleground state of Wisconsin, Ms Harris likened her Republican opponent to fraudsters she said she had prosecuted.

Trump, meanwhile, assailed her record on the border, and posted on social media: “Lyin’ Kamala Harris destroys everything she touches!”

It comes a day after she secured the support of a majority of Democratic delegates, paving the way for her to become the party's nominee.

On Sunday afternoon, President Joe Biden announced he was withdrawing from the race and endorsed his vice-president amid mounting pressure from top Democrats and donors following his disastrous debate against Trump in late June.

The fledgling Harris campaign raised a staggering $100m plus (£77m) in the 36 hours after Mr Biden’s exit.

Adding to her momentum, a new national poll from Reuters and Ipsos shows her with a two-point lead over Trump, 44% to 42%.

Taking the stage to applause at a high school in a suburb of Milwaukee on Tuesday, Ms Harris highlighted her experience as California's attorney general.

"I took on perpetrators of all kinds,” she said. “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," Ms Harris added. "In this campaign, I promise you, I will proudly put my record against his any day of the week."

In response, the crowd shouted "Kamala! Kamala!" Some observers noted that the audience's enthusiasm contrasted with that seen at Biden events this electoral cycle.

When her Republican opponent's name was mentioned, many attendees chanted "lock him up", echoing a similar refrain at Trump events when he was running against Hillary Clinton in 2016.

Trump, meanwhile, posted on his social media platform, Truth Social, about a poll finding that Ms Harris was the most unpopular vice-president in US history.

He also shared a post noting that she was rated by the non-partisan congressional scorekeeper GovTrack as one of the most left-wing among dozens of Democratic senators during her tenure.

Ms Harris laid out a number of liberal priorities in her speech, on gun control and abortion access, as well as child poverty, union rights and affordable healthcare.

"Do we want to live in a country of freedom, compassion and rule of law, or a country of chaos, fear and hate?" she said.

Whether Ms Harris can maintain her momentum is unclear. In a memo released Tuesday, pollster Tony Fabrizio predicted her "honeymoon" period with voters would end and that there would be a "refocus on her role as Biden's partner and co-pilot".

The Trump campaign is attacking Ms Harris' "failure" to stem a record influx of illegal immigrants at the US-Mexico border. It has also signalled that it will slam the Biden-Harris administration's record on crime and inflation.