As they walked out for the first time as presidential running mates on Tuesday night, Tim Walz turned to Kamala Harris and mouthed one word: “wow”.

It spoke to the enthusiastic response from the Philadelphia crowd, but also reflected the unlikely journey that the Minnesota governor has been on over the past week.

Few people had Mr Walz on their early lists of possible vice-presidential choices. But on Tuesday, the dark horse won 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.

All of this was on display here in Philadelphia.

After noting that violent crime rates went up under Donald Trump, he added – with a smile - “and that doesn’t even count the crimes he committed”. He called the Republican ticket “weird as hell” –a label that has become a Democratic mantra in just a matter of days. And on the topic of abortion, he said government should follow a midwestern golden rule: “Mind your own damn business”.

Mixing humour with jabs - and speaking openly of the “joy” he sees in Democratic politics - may prove to be a more effective way to convince undecided voters who were simply not convinced by the dark “threat to democracy” rhetoric the Biden campaign had been using.

Mr Walz’s aw-shucks affability 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.

Mr Shapiro served as the warm-up act for the new Democratic ticket, and he received a hero’s welcome from his home-state crowd. It was a reminder of what Ms Harris passed over in picking Mr Walz – a popular politician with a silver tongue from perhaps the most important state on the electoral map.

Mr Walz was a safer pick than the Pennsylvanian, however, whose criticism of pro-Palestinian protesters and support for using public funds for private schools prompted objections from key parts of the Democratic base. These risked reopening intraparty divisions at a time when Democrats were finally pulling together.

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 ultimately help decide this election.

By taking a Republican-held House seat in 2006, Mr Walz 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.

He’s also a native of Nebraska, which in 2020 delivered one of its electoral votes to Joe Biden. It’s by far the smallest battleground, but in a close race it could be the difference between victory and defeat.

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”.

They point to his record in Minnesota of enacting left-wing social programmes and accuse him of not doing enough to control the demonstrations that broke out after the death of George Floyd at the hands of Minneapolis police officers in 2020.

At the very least, Republicans may welcome not having to face-off against Mr Shapiro, who has a more centrist profile and might have given Ms Harris a decisive boost in Pennsylvania.

JD Vance, Mr Walz's Republican adversary for the vice-presidency, said the choice showed Ms Harris was willing to "bend the knee to the most radical elements of her party".

Trump, meanwhile, said Mr Walz will unleash "HELL ON EARTH and open our borders to the worst criminals imaginable".

But even if Mr Walz provides a more inviting target for Republicans, making that rhetoric stick on his friendly, meat-and-potatoes persona will be no easy task for the Trump campaign.

Now the newly minted Democratic ticket hits the campaign trail, with 91 days left until election day.

“That’s easy,” Mr Walz said of the three-month home stretch. “We can sleep when we’re dead”.