Ms Kamala Harris and Mr Tim Walz have just launched a five-day tour of key battleground states. Photo: BBC News

At the event in Philadelphia on Tuesday night, the party’s new nominee for vice-president said their Republican rivals in November's election were "weird as hell".

The Minnesota governor spoke in front of thousands of supporters just hours after he was announced as Ms Harris’s pick for the role.

The Trump campaign, meanwhile, was quick to attack Mr Walz as a "dangerously liberal extremist".

The 60-year-old is billed as someone who could win back rural and working-class voters who have gravitated to Donald Trump in crucial midwestern states.

At the rally in the key swing state of Pennsylvania, Ms Harris, currently the US vice-president, said she and Mr Walz were the "underdogs" in what is expected to be a close election but had the momentum.

She introduced her running mate as “a fighter for the middle class, a patriot”.

Mr Walz then recounted his small-town roots in Nebraska and his career as a national guardsman and teacher, before attempting to draw a contrast with Trump.

"He doesn’t know the first thing about service - because he's too busy serving himself," said the former army sergeant and football coach.

He got some of the loudest cheers of the night when he took aim at the former president's criminal record, with chants of “lock him up” from those in the arena.

He also sparked applause when he invoked a viral attack line that caught the eye of the Harris campaign as she considered who would be her running mate.

"These guys are creepy and, yes, just weird as hell," Mr Walz said of their Republican challengers.

The pair mostly pitched themselves as defenders of individual freedoms, including on abortion rights and safety from gun violence.

Mr Walz showed the plain-spoken, folksy style that has won praise from Democrats, as he took a jab at Republicans on the issue of abortion access.

“Mind your own damn business!" he said, drawing an ovation from the crowd of more than 10,000 at Temple University.

Ms Harris and Mr Walz have just launched a five-day tour of key battleground states.

They will also speak at the Democratic National Convention, which runs from 19 to 22 August in Chicago.