Tim Walz highlighted his rural background and criticized Donald Trump for wanting to take the US “backwards” during his first appearance as Kamala Harris’s running mate at a lively Democratic Party rally.

Speaking at the Philadelphia event on Tuesday night, the newly announced vice-presidential nominee described the Republican opponents in the upcoming November election as “weird as hell.”

The Minnesota governor addressed thousands of enthusiastic supporters just hours after being announced as Ms. Harris’s choice for the vice-presidential role.

The Trump campaign swiftly labeled Mr. Walz as a “dangerously liberal extremist.”

At 60 years old, Walz is seen as a candidate who can appeal to rural and working-class voters who have leaned towards Donald Trump in critical Midwestern states.

During the rally in the key swing state of Pennsylvania, Ms. Harris, the current US vice president, described herself and Mr. Walz as “underdogs” in what is expected to be a tight race but emphasized their growing momentum. She introduced Walz as “a fighter for the middle class, a patriot.”

Mr. Walz shared his small-town upbringing in Nebraska and his career as a national guardsman and teacher, aiming to contrast himself 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.

The crowd erupted in cheers when he criticized the former president’s criminal record, with chants of “lock him up” filling the arena.