Los Angeles, California - Tim Walz made his first solo outing as a vice-presidential candidate on Tuesday as the Democratic ticket looks to build momentum in the quickening US election campaign.

The folksy Midwesterner, who was catapulted to national attention a week ago when Kamala Harris picked him as her running mate, addressed union organizers in Los Angeles, pledging he would be part of a pro-worker administration.

"The Vice President and I, we know exactly who built this country; it was nurses, it was teachers, and it was state and local government employees that built this nation," he told cheering members of the American Federation of State, County and Municipal Employees.

"It's not just a saying, it's a fact: when unions are strong, America is strong."

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The 60-year-old governor of Minnesota drew a contrast with Republican rivals Donald Trump and JD Vance, who he said "see the world very differently."

"The only thing those two guys knows about working people is how to work to take advantage of them," he said.

But in a shout-out to moderate Republicans wary of Trump's hard-right positions, he also invoked a hero of the party, noting that he was the "first union member on a presidential ticket since Ronald Reagan."

Walz's high-energy likeability is quickly becoming a huge selling point for the Democratic Party, whose fortunes have been transformed since President Joe Biden dropped out of the race last month.

The one-time public school teacher, football coach, and military veteran has a demeanor that reminds many Americans of an idealized version of a dad.

To the delight of the party faithful, he also has a way with words that has seen him successfully brand Trump and Vance as "weird."

Vance, meanwhile, has been leading a wave of Republican attacks on Walz over the timing of his military retirement.

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Walz's selection is expected to bolster support among white working-class men, a constituency that will be key to victory in battleground states like Wisconsin, Michigan, and Pennsylvania.

He and Harris visited all three states last week, plus Arizona and Nevada, as part of a rapid blitz of campaign rallies together.

At the Los Angeles gathering on Tuesday, union member Henry Garrido, who works for New York City schools, said he found the governor relatable.

"He's down to earth," he told AFP. "He's one of us. He's somebody who's been there, who's been a teacher, has been in the lunchroom."