Kamala Harris, the de facto Democratic nominee for US president, has named Tim Walz, the governor of Minnesota, as her running mate ahead of the November election.

The decision ends intense speculation over which candidate Harris would pick to go up against Donald Trump, the Republican nominee and former president, and his choice for vice-president, the Ohio senator JD Vance.

Walz first ran for office in 2006 in a Republican-leaning congressional district, upsetting the incumbent. He kept the seat until he won the Minnesota governorship in 2018, then again in 2022. Under his leadership, the state has seen significant progressive legislative wins in recent years, including universal school meals, legalized marijuana, abortion protections and gun control measures.

Before he entered public office, he was a school teacher in Mankato, Minnesota, teaching geography to high school students. He also served in the army national guard for 24 years.

In an Instagram post announcing the pick, Harris said: “One of the things that stood out to me about Tim is how his convictions on fighting for middle-class families run deep. It’s personal.”

She cited his upbringing in Nebraska, and how after his father’s death to cancer, his family relied on social security survivor benefits to make ends meet. He used the GI bill to attend college. He coached high school football and advised the high school’s gay-straight alliance. His background is “impressive in its own right”, but also informs his governing, she said.

Minnesota Democrats’ legislative record played into her choice – she noted a law that constitutionally protects access to abortion and one requiring universal background checks for gun purchases.

“But what impressed me most about Tim is his deep commitment to his family,” she added.

“We are going to build a great partnership. We are going to build a great team. We are going to win this election.”

Walz posted a short statement to X on Tuesday. “Vice President Harris is showing us the politics of what’s possible. It reminds me a bit of the first day of school,” he said.

His midwesterner dad charm and straight-talk propelled him up the list as a potential vice-presidential pick, though, and as the head of the Democratic Governors Association, he has been stumping for Biden and Harris for the past year.

It was his simple retort against Trump and his allies that caught national Democrats’ attention most: he called them weird. His clips on TV shows went viral, showing him pushing back on Republicans’ “weird behavior” while showcasing a list of what he had accomplished as a Democratic governor and how Democrats would govern if they win the White House again.

Walz explained in a TV interview why he had started calling Trump weird. It’s true that Trump’s policy would put women’s lives on the line and that he’s a threat to constitutional values, Walz said. But he’s also on the campaign trail “talking about Hannibal Lecter and shocking sharks and just whatever crazy thing pops into his mind”.

“Have you ever seen the guy laugh? That seems very weird to me, that an adult can go through six and a half years of being in the public eye. If he has laughed, it’s at someone, not with someone. That is weird behavior,” Walz said of Trump.

Walz grew up in small-town Nebraska, giving him rural bona fides that will help voters who have moved away from Democrats in recent years.

“The golden rule that makes small towns work so we’re not at each others’ throats all the time in a little town is: mind your own damn business,” Walz said in one TV spot.

His former colleagues praised his ability to connect with those crucial voters in the Rust belt, and to not only explain what’s bad about Republicans, but what Democrats would actually do in office.

Tim Ryan, a former Democratic US representative and Walz’s friend, called to mind a recent clip in which Walz mentioned that Minnesota ranked in the top three for happiest states in the nation. “Isn’t that really the goal here? For some joy? When he mentioned that I was like, dang man, that’s really good. That’s really good, because it gets us out of the political space and into the human being space.”

Some political commentators had suggested that, as the first woman of color nominated by a major party, Harris was mostly likely to pick a white man to balance the ticket.

The 59-year-old former California senator is looking to build on a successful campaign launch after stepping in to replace Joe Biden, who bowed to pressure from Democratic colleagues and dropped out of the race after a disastrous debate performance against Trump.

Harris and Walz can expect a rapturous welcome at the Democratic national convention in Chicago starting on 19 August. She has been endorsed by former presidents Barack Obama and Bill Clinton.

A New York Times/Siena College national opinion poll published on 25 July found that Harris has narrowed what had been a sizable Trump lead. Trump was ahead of Harris 48% to 46% among registered voters, compared with a lead of 49% to 41% over Biden in early July.

Republicans immediately began attacking Walz as a “radical leftist” and claimed that picking Walz was “a massive gift to Republicans”, suggesting his presence on the ticket will tank the Harris campaign.

In a statement, the Trump campaign called Walz a “west coast wannabe” who has “spent his governorship trying to reshape Minnesota in the image of the Golden state”. The campaign brought up a clip from 2017 where Walz talks about electoral maps that show broad swathes of red, saying those areas are “mostly cows and rocks”.

“If Walz won’t tell voters the truth, we will: just like Kamala Harris, Tim Walz is a dangerously liberal extremist, and the Harris-Walz California dream is every American’s nightmare,” the Trump campaign said.

Speaking in Philadelphia on Tuesday, Vance first told reporters he could not “say a lot about who Tim Walz is because the Democrats have shown a willingness to put a little switcheroo on us”, by switching presidential candidate from Biden to Harris.

Vance said he had called Walz to congratulate him on his selection and would debate him, but only after the Democratic convention later this month.

“So that’s the first reason,” for not commenting too much about Walz, Vance said – though he subsequently attacked Walz freely in terms he himself indicated: “The second reason is that Tim Walz’s record is a joke. He’s one of the most far-left radicals in the entire United States government, at any level.”