PHILADELPHIA: Kamala Harris made her debut Tuesday with new White House running mate Tim Walz, who unleashed a stinging rebuke of their Republican rival Donald Trump, questioning his commitment to the country and skewering his record in office.

Hours after being announced on the Democratic ticket, the Minnesota governor took the stage with Vice President Harris in the crucial swing state of Pennsylvania, and signaled his intent to press the case aggressively against Trump.

"Donald Trump - he sees the world differently. He doesn't know the first thing about service because he's too busy serving himself," Walz told a crowd of around 10,000 wildly cheering supporters in Philadelphia.

"He accused Trump of deliberately weakening the economy for his own ends, mocking law and order, and sowing "chaos and division"

"He drove our economy into the ground, and make no mistake: violent crime was up under Donald Trump. That's not even counting the crimes he committed," Walz added, earning roars of laughter and boisterous applause.

With less than three months to go until election day, most Americans don't know who Walz is - and he gave voters a glimpse of his personal story, describing his military service, his experiences as a public school teacher and his record in politics.

He spoke of his upbringing in the small town of Butte, Nebraska where he worked on the family farm and where community was a "way of life" with neighbors striving together "for the common good."

In their first campaign appearance together, Harris and Walz offered a glimpse of how they might appeal to voters as a duo - one a trailblazing Black and South Asian former senator from California, the other a white ex-congressman from the blue-collar US heartland.

Harris, 59, described Walz as "a leader who will help unite our nation and move us forward, a fighter for the middle class, a patriot who believes, as I do, in the extraordinary promise of America.

"A former attorney general of California, Harris has sought to draw a contrast between her past as a prosecutor and Trump's criminal record, which extends to 34 felonies, with over a dozen more charges pending.

But she told the crowd: "This campaign - our campaign - is not just a fight against Donald Trump.

Our campaign - this campaign - is a fight for the future."Walz's role as Harris's top cheerleader also started coming into focus as he lauded her work and career.

"She took on the predators and the fraudsters, she took down transnational gangs, stood up against powerful corporate interests," Walz said.

"And she never hesitated to reach across the aisle if it meant improving people's lives... And she does it all with a sense of joy."

Walz's recent viral success in distilling Democrats' attack lines against Republicans into the one-word characterization - "weird" - propelled him up Harris's shortlist, and he got cheers for reprising it in Philadelphia.

Pennsylvania Governor Josh Shapiro, who was part of the undercard in Philadelphia and spoke glowingly of Walz, had been a favorite but Walz, 60, was said to have connected more deeply with Harris during interviews.

The vice president's first joint appearance with her new running mate comes before the Democratic Party convention in Chicago, starting Aug 19. The rollout of his candidacy in Pennsylvania was no accident.

The state is a key battleground, and part of the "blue wall," along with Michigan and Wisconsin, that powered President Joe Biden to victory in 2020.

The noisy rally provided a striking reminder of how different the race looked just a month ago, before 81-year-old Biden bowed to mounting concerns over his age and exited the race, endorsing Harris.

She has since obliterated fundraising records on her way to erasing Trump's lead in polls over Biden.Philadelphia was the start of a multi-day tour for Harris and Walz through the swing states where the race is likely to be decided, taking in Wisconsin, Michigan, Arizona and Nevada.

Republicans are scrambling to craft a narrative around Walz as a "dangerously liberal extremist" while Trump's vice presidential pick J.D. Vance suggested that Harris - who is married to a Jewish man - passed over Shapiro because of anti-Semitism.