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Watch key moments from first Harris-Walz campaign rally
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."
President Joe Biden, who suspended his own election campaign last month and endorsed Ms Harris, said in a statement that the new Democratic ticket "will be the strongest defenders of our personal freedoms and our democracy".
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.
He has described his entry into politics as almost accidental. He said he volunteered for John Kerry's 2004 presidential campaign after being angered by an incident in which two of his pupils were turned away from a George W Bush campaign event.