Few people had Tim Walz on their early lists of possible vice-presidential choices. But sometimes the dark horse wins the race.

In a year when “vibes” have been everything in politics – on the economy, on the campaign trail – that is exactly what Kamala Harris has gone for: good vibes.

The Minnesota governor has a “midwestern nice” appeal, even when he is throwing political punches. His background – a teacher, a football coach, an Army National Guard enlisted soldier – broadcasts “meat-and-potatoes middle America”, as does perhaps his balding, rotund, slightly dishevelled appearance.

His affable style and ability to land rhetorical blows on Donald Trump without seeming too aggressive have also brought him to national attention.

The “weird” label he applied to the former president and his running mate, JD Vance, became a Democratic mantra in just a matter of days – and it is now a standard part of Ms Harris’s stump speech.

It may prove to be a more effective way to win over undecided voters who were simply not convinced by the dark “threat to democracy” rhetoric the Biden campaign had been using.

Mr Walz stood in sharp contrast with other possible choices – the polished and ambitious Pennsylvania Governor Josh Shapiro and Arizona Senator Mark Kelly, with his arrow-straight military demeanour.

He is also a safer pick than the other two, who each held some positions that risked angering portions of the Democratic base.

And while Minnesota is not a battleground state, the Harris campaign may hope that Mr Walz has midwestern appeal in places like Wisconsin and Michigan, which will help decide this election.

By taking a Republican-held House seat in 2006, he has already shown he can win round a significant number of rural and Republican voters.

And Mr Walz has proven adept at defending his record of progressive legislation in a way that moderates and independent voters can understand.

• PROFILE: The ex-football coach and teacher – now VP pick
• ANALYSIS: Three ways Trump will try to end Harris honeymoon
• SWING STATES: Where the election could be won and lost
• EXPLAINER: RFK Jr and others running for president
• VOTERS: US workers in debt to buy groceries

Nancy Pelosi, the Democrat grandee who was so instrumental in persuading Joe Biden to step aside for Ms Harris, has been gushing in her praise of the “wonderful” Mr Walz.

It is no surprise. His 2006 victory helped deliver the House majority to Ms Pelosi as House Speaker, and to the Democrats for the first time in 12 years.

Republicans are going to try to erase these early good vibes and replace them with a darker picture.

The Trump campaign has already branded him a “dangerously liberal extremist” and a “far-left lunatic”.

JD Vance, his Republican adversary for the vice-presidency, said the choice shows Ms Harris is willing to “bend the knee to the most radical elements of her party”.

Trump claims Mr Walz will unleash “HELL ON EARTH and open our borders to the worst criminals imaginable”.

But making that rhetoric stick on such an affable, meat-and-potatoes persona? Republicans may have their work cut out.