Kamala Harris has introduced Minnesota governor Tim Walz as her running mate, turning to an affable longtime politician who Democrats hope can keep newfound party unity alive in a presidential campaign heading towards Election Day.

The two made their first public appearance at a rally in Philadelphia just hours after Ms Harris announced vice presidential pick to take on the Republican ticket of former president Donald Trump and Ohio senator JD Vance.

“Since the day that I announced my candidacy, I set out to find a partner who can help build this brighter future,” she said. “I’m here today because I’ve found such a leader, governor Tim Walz of the great state of Minnesota.”

In choosing the 60-year-old Mr Walz, Ms Harris is elevating a Midwestern governor, military veteran and union supporter who helped enact an ambitious Democratic agenda for his state, including sweeping protections for abortion rights and generous aid to families.

It was her biggest decision yet as the Democratic nominee and she went with a broadly palatable choice — someone who says politics should have more joy and who deflects dark rhetoric from Republicans with a lighter touch, a strategy the campaign has been increasingly turning to since Ms Harris took over the top spot.

“He’s going to be a great vice president,” she said while boarding Air Force Two.

Mr Walz is joining Ms Harris on the ticket during one of the most turbulent periods in modern US politics. Republicans have rallied around Mr Trump after he was targeted in an attempted assassination in July.

Just days later, President Joe Biden ended his re-election campaign, forcing Ms Harris to scramble to unify Democrats and decide on a running mate over a breakneck two-week stretch.

She hopes Mr Walz will help her shore up her campaign’s standing across the upper Midwest, a critical region in presidential politics that often serves as a buffer for Democrats seeking the White House.

The party remains haunted by Mr Trump’s wins in Michigan and Wisconsin in 2016. He lost those states in 2020 but has zeroed in on them as he aims to return to the presidency this year and is expanding his focus to Minnesota.

Since Mr Walz was announced, the team has raised more than 10 million dollars (£7.8 million) from grassroots donations, the campaign said.

Mr Walz is far from a household name. An ABC News/Ipsos survey conducted before he was selected but after vetting began showed that nearly nine in 10 US adults did not know enough to have an opinion about him.

Ms Harris, the first black woman and person of south Asian descent to lead a major party ticket, initially considered nearly a dozen candidates before zeroing in on a handful of serious contenders.

Mr Trump has focused much of his campaign on appealing to men, emphasising strength in national leadership and even featuring the wrestler Hulk Hogan on the final night of the Republican National Convention.

Ms Harris’s finalists — all white men — marked an acknowledgement of the Democrat’s need to at least try to win over some of that demographic.

She personally interviewed three finalists: Pennsylvania governor Josh Shapiro, Arizona senator Mark Kelly and Mr Walz. She wanted someone with executive experience who could be a governing partner, and Mr Walz also offered appeal to the widest swathe of the diverse coalition.

His selection drew praise from legislators as ideologically diverse as progressive leader, representative Alexandria Ocasio-Cortez, and independent senator Joe Manchin, a moderate who left the Democratic Party earlier this year.

A team of lawyers and political operatives led by former attorney general Eric Holder pored over documents and conducted interviews with potential selections. Ms Harris mulled the decision over on Monday with senior aides and finalised it on Tuesday morning, according to sources.

Mr Shapiro, an ambitious politician in his own right, struggled with the idea of being number two at the White House and said he felt he had more to do in Pennsylvania, according to one source. There was also public pushback to Mr Shapiro for his stance on Israel from Arab American groups and younger voters angry over the administration’s response to the Israel-Hamas war.

The other contenders threw their support behind the ticket on Tuesday, and Mr Shapiro was one of the speakers at Tuesday’s Philadelphia rally. Mr Biden described the Harris-Walz ticket as “a powerful voice for working people and America’s great middle class”.

Mr Walz coined one of the Democrats’ buzziest campaign bits to date, calling Mr Trump and Mr Vance “just weird”, a label that the Democratic Governors Association — of which Mr Walz is chairman — amplified in a post on X and Democrats have echoed more broadly.

During a fundraiser for Ms Harris on Monday in Minneapolis, Mr Walz said: “It wasn’t a slur to call these guys weird. It was an observation.”

Ms Harris and Mr Walz will spend the next five days touring critical battleground states, visiting Wisconsin and Michigan on Wednesday and Arizona and Nevada later in the week.

Mr Vance, for his part, plans stops in some of the same areas. He said on Tuesday that he had called Mr Walz earlier in the day and left a voice message.

The Trump campaign on Tuesday immediately tried to tag Walz as a far-left liberal.

“It’s no surprise that San Francisco liberal Kamala Harris wants West Coast wannabe Tim Walz as her running-mate – Walz has spent his governorship trying to reshape Minnesota in the image of the Golden State,” said Karoline Leavitt, Mr Trump’s campaign press secretary.

“Walz is obsessed with spreading California’s dangerously liberal agenda far and wide.”

Mr Walz, who grew up in the small town of West Point, Nebraska, was a social studies teacher, American football coach and union member at Mankato West High School in Minnesota before entering politics.

He won the first of six terms in Congress in 2006 from a mostly rural southern Minnesota district and used the office to champion veterans issues.

He served 24 years in the Army National Guard, rising to command sergeant major, one of the highest enlisted ranks in the military, although he did not complete all the training before he retired so his rank for benefits purposes was set at master sergeant.

He ran for governor in 2018 on the theme of One Minnesota and won by more than 11 points.