US presidential hopeful Kamala Harris and her newly announced running mate, Minnesota Governor Tim Walz, came out swinging against former President Donald Trump and his running mate during their first joint campaign appearance on Tuesday.

"Donald Trump — he sees the world differently," Walz told the audience in the 10,000-capacity arena in Philadelphia, the largest city in the state of Pennsylvania. "He doesn't know the first thing about service because he's too busy serving himself," he said.

The governor used the rally to introduce himself, recounting his childhood working on a farm in Nebraska, his military service, and his time in Congress.

Trump 'mocks our laws, sows chaos and division,' says Walz

Walz offered a stinging rebuke of Trump and Ohio Senator JD Vance, Trump's vice presidential pick.

"He mocks our laws, he sows chaos and division, and that's to say nothing of his record as president. He froze in the face of the COVID crisis, he drove our economy into the ground, and make no mistake, violent crime was up under Donald Trump."

"And that’s not even counting the crimes he committed," Walz added, in reference to the former president's 34 felony convictions and other charges.

Taking swipes at Vance, Walz said his GOP rival and Trump "are creepy and yes, they're weird as hell."

"I can't wait to debate the guy," Walz added.

Trump has been hesitant to commit to a television debate with Harris.

Unlike Trump, Vance has displayed more conviction over a potential clash, saying earlier on Tuesday that he "absolutely" wants to debate Walz.

Harris makes the case for her running mate

Harris praised Walz's accomplishments, telling the audience in Philadelphia: "Since the day that I announced my candidacy, I set out to find a partner who can help build this brighter future."

While standing alongside her running mate at the the Liacouras Center at Temple University, Harris said that that Walz is "the kind of person who makes people feel like they belong and then inspires them to dream big ... That's the kind of vice president America deserves."

Josh Shapiro backs Harris-Walz ticket

Pennsylvania Governor Josh Shapiro, who was seen as a contender to join Harris as her running mate, kicked off the rally on Tuesday night in his home state with praise for both candidates.

He criticized Republicans and promised 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."

Additionally, Harris officially secured the Democratic nomination on Tuesday, following a week of virtual roll call in which she received 99% of the votes of all the delegates who participated.

This solidifies the Harris-Walz campaign to go up against the Trump-Vance ticket for the US election contest this November 5.

rm/jsi (AFP, AP)