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 raucous Democratic Party rally.
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 governor also said he "can't wait to debate" Republican vice-presidential nominee JD Vance, "that is, if he's willing to get off the couch and show up".
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.
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 progressive agenda.
Democrats have used control of the state legislature to guarantee abortion rights, pass gun control measures and institute paid family leave.
Republicans have criticised Mr Walz for Minnesota's mask mandate and a shutdown of businesses and schools during the Covid-19 pandemic, as well as 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 Mr Vance, Trump’s running mate, who assailed the new 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."
President Joe Biden, who suspended his own election campaign last month and endorsed Ms Harris, said in a statement that the Democratic ticket "will be the strongest defenders of our personal freedoms and our democracy".
Another of the finalists to be Ms Harris’s running mate, Pennsylvania Governor Josh Shapiro, was also at the rally.
He had faced sharp criticism from the left over his support for Israel and his handling of college protests sparked by the war in Gaza.
Some Trump advisers have expressed relief that Ms Harris did not pick Mr Shapiro because of concerns he could help deliver the pivotal state of Pennsylvania.
Before entering the governor's office, Mr Walz represented a Republican-leaning district in the US Congress for 12 years.
He won that seat in 2006 - the only Democrat to have done so in the mostly rural district over the past three decades.
Mr Walz is a native of Nebraska and the son of a school administrator and a stay-at-home mother.
He grew up farming and hunting and served in the Army National Guard for 24 years after joining aged 17.
The young Mr Walz also taught secondary school pupils - first for a year in China, a country he says he has visited about 30 times. He speaks some Mandarin.
His wife, Gwen Whipple, a fellow teacher, drew him to her native Minnesota, where he taught social studies and geography and coached American football.
