Tim Walz touted his rural roots and said Donald Trump would take the US “backwards”, as he appeared for the first time as Kamala Harris’s running mate.
At a rally in Philadelphia, the Democratic nominee for vice-president said their Republican rivals in November’s election were “weird as hell”.
The Trump campaign said Mr Walz, the current Minnesota governor, was a “dangerously liberal extremist”.
Mr Walz, 60, is billed as someone who can win back rural and working-class voters who have gravitated to Trump in crucial midwestern states.
At Tuesday night’s 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.
Mr Walz told a packed arena in Philadelphia that Trump “sees the world differently”.
“He doesn’t know the first thing about service – because he’s too busy serving himself,” said the former army sergeant and football coach.
To cheers, he also 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.
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.
As the current two-term governor of Minnesota, Mr Walz has overseen one of the most productive legislative periods in state history, implementing a sweeping left-wing agenda.
Democrats have used control of the state legislature to guarantee abortion rights, pass gun control measures and institute paid family leave, among other things.
Republicans have criticised Mr Walz for Minnesota’s mask mandate and shutdown of businesses and schools during the Covid-19 pandemi,c and his delay in deploying the National Guard to deal with rioting after George Floyd’s murder in 2020.
Also in Philadelphia earlier on Tuesday was JD Vance, Trump’s vice-presidential nominee, who assailed the Democratic White House ticket.
The Ohio senator told reporters that Ms Harris’s choice of Mr Walz shows that “when given the opportunity she will bend the knee to the most radical elements of her party.”
Trump’s campaign said in a statement: “Just like Kamala Harris, Tim Walz is a dangerously liberal extremist, and the Harris-Walz California dream is every American’s nightmare.”