New Democrat running mate Tim Walz has taken a dig at Donald Trump in his first appearance with presidential nominee Kamala Harris.

Harris and Walz, the Minnesota Governor, appeared together for the first time at a boisterous rally in Philadelphia on Tuesday.

They stepped out to rousing cheers from the audience of 10,000 – the biggest yet for Harris, or even President Joe Biden before he dipped out of November’s election race.

“What direction will this country go in? Donald Trump would damn sure take us backwards, let’s be clear about that,” Walz said.

He used the rally to introduce himself to Americans – Walz is a former high school teacher and football coach, an Army National Guard veteran and a former congressman. There were also more digs for Trump and his Republican vice-presidential candidate, JD Vance.

“Make no mistake. Violent crime was up under Donald Trump. And that’s not even counting the crimes he committed,” Walz said, a swipe at the former president’s 34 felony convictions and other charges.

And he repeated the claim that helped put him in the national spotlight.

“You know it, you feel it. These guys are creepy, and yes, just weird as hell,” Walz said of Republicans.

Introducing Walz, Harris predicted he would have a new title in November: Vice-president of the US.

“He’s the kind of person who makes people feel like they belong and then inspires them to dream big,” she said.

She announced her choice of Walz earlier on Tuesday, opting for a vice-presidential running mate with executive experience, military service and a track record of winning over the rural, white voters who have gravitated to Trump over the years.

Pennsylvania, the site of their first rally, is seen as perhaps the most critical state in what is expected to be a close election between the Democrats and Trump and Vance.

Walz was elected to a Republican-leaning district in the US House of Representatives in 2006 and served 12 years before being elected governor of Minnesota in 2018 and again in 2022.

He has pushed a progressive agenda that includes free school meals, goals for tackling climate change, tax cuts for the middle class and expanded paid leave for workers.

“As a governor, a coach, a teacher and a veteran, he’s delivered for working families like his own,” Harris said in a statement released by her campaign.

Trump and Vance were quick to criticise the new competition as too liberal.

“This is the most Radical Left duo in American history,” Trump wrote on his social media platform.

Vance knocked Walz for his handling of protests after George Floyd, a black man, was killed in Minneapolis by a white police officer in 2020, saying Walz was not assertive enough in combating the rioters.

“The biggest problem with the Tim Walz pick – it’s not Tim Walz himself. It’s what it says about Kamala Harris, that when given the opportunity she will bend the knee to the most radical elements of her party,” Vance said in Philadelphia.

Americans typically focus on the person at the top of the ticket when choosing whom to vote for. But vice-presidential candidates can help or hurt their running mates based on their backgrounds, home state popularity and ability to sway important constituencies or independent voters.

“She went with her gut on this one and chose the option that won’t alienate young folks,” Republican strategist Rina Shah said.

Walz beat Pennsylvania’s popular governor, Josh Shapiro, for the No.2 role. Shapiro had faced sharp criticism from the left, especially progressive groups and pro-Palestinian activists, over his support for Israel and his handling of college protests sparked by the war in Gaza.

Shapiro delivered a fiery speech at the Tuesday night rally in his home state, attacking Republicans and promising to “work my tail off” to get Harris elected. He also offered a strong endorsement of Walz, telling the crowd that he is an “outstanding governor” and a “great patriot”.

Harris and Walz will seek to build on the momentum that she has sustained since becoming the Democratic Party’s standard bearer after Biden ended his re-election campaign last month. Harris has raised hundreds of millions of dollars and reshaped the race against Trump.

“My promise to you is this: Our campaign will reach out to everyone. From red states to blue states. From the heartland to the coasts,” she told the crowd in Philadelphia.

“We are running a campaign on behalf of all Americans and, if elected, we will govern on behalf of all Americans.”

After their first joint appearance, Harris and Walz plan a multi-city tour of critical swing states, including Wisconsin, Arizona and Nevada.

-with AAP