At the gathering in Philadelphia on Tuesday night, the party’s new vice-presidential contender said their Republican opponents in the November election were “weird as hell”.
Just hours after being named Ms Harris’ candidate for the position, the Minnesota governor addressed thousands of supporters.
Meanwhile, the Trump team labeled Mr Walz a “dangerously liberal extremist”.
The 60-year-old is portrayed as someone who can reclaim rural and working-class supporters who have flocked to Donald Trump in key midwestern states.
At the gathering in Pennsylvania, Ms Harris, the current US vice president, stated that she and Mr Walz were the “underdogs” in what is likely to be a close election but had momentum.
She described her running mate as “a fighter for the middle class, a patriot”.
Mr Walz then spoke about his small-town upbringing in Nebraska, as well as his career as a national guardsman and teacher, before attempting to establish a contrast with Trump.
“He doesn’t know the first thing about service – because he’s too busy serving himself,” claimed the former army sergeant turned football coach.
He drew some of the greatest cheers of the night when he took aim at the previous president’s criminal past, with chanting of “lock him up”.
