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 Harris’s pick for the role.
The Trump campaign, meanwhile, was quick to attack 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, 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.
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 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.