US presidential hopeful Kamala Harris and her freshly selected running mate Tim Walz hit the campaign trail together Tuesday for the first time, as the Democratic duo fired up supporters at a rally in key battleground Pennsylvania.

Walz, the Minnesota governor whose everyman credentials could help win over voters, joined Harris in Philadelphia to launch a tour through four swing states where the race against Donald Trump is likely to be decided.

Aiming to make history as the first woman US president, Harris -- already a trailblazer as the first female and first Black and South Asian vice president -- has little time before Election Day on November 5.

"We fight for a future where we defend our most fundamental freedoms," Harris told cheering supporters as Walz stood beside her.

"I set out to find a partner who can help build this brighter future. A leader who will 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.

"I'm here today because I've found such a leader: Governor Tim Walz."

President Joe Biden earlier hailed his deputy's "great decision," saying she and Walz "will be a powerful voice for working people and America's great middle class."

Expectations had always been that Harris would pick a white man to balance the ticket and help draw working-class, white male voters, a demographic that propelled Trump to victory in 2016.

Walz fits that description as a 60-year-old Midwesterner with a folksy manner, decades of military experience and a rural perspective.

He dismisses the "weird" positions of Trump and Republican running mate J.D. Vance, and he is also a gun owner who tweets about hunting, goes ice fishing and rides rollercoasters with his daughter.

The onetime teacher and school sports coach flipped a Republican district in 2006 to win a US House seat from a state seen as light years from the coastal elites of California, Harris's home turf, or the East Coast.

At the same time, Walz will appeal to the left for championing cannabis legalization, worker protections, abortion rights and tighter firearm restrictions.

-'He's relatable'

Trump's campaign branded the Minnesotan a "dangerously liberal extremist" seeking to impose California values on the nation.

But supporters gathered at the Philadelphia rally told AFP they were excited about father-of-two Walz on the ticket.

"He's relatable," Carlos Ruiz, a 39-year-old corrections officer, said of Walz, adding that "his word 'weird' has really resonated with the country in a special way."

Narrating an introductory video released by the campaign, Walz described growing up in Nebraska and absorbing the values of "community" and "respect, compromise, service to country."

Walz earned enthusiastic endorsements from Democratic liberals as well as centrists like Senator Joe Manchin, who called him "the real deal."

After being named by Harris, Walz said it was the "honor of a lifetime" and quipped it felt like "the first day of school."

It has been a remarkable journey for Harris, who only entered the race last month when 81-year-old Biden withdrew, bowing to mounting concerns over his age.

The former prosecutor and senator has since obliterated fundraising records on her way to erasing what had been Trump's growing lead in polls over Biden.

The latest University of Massachusetts Amherst poll has Harris leading Trump nationally by three percentage points, compared to a four-point Trump lead over Biden in January.

In the swing states Harris and ex-president Trump, who lost to Biden in 2020, are neck and neck.

Campaign test

Picking a running mate was seen as the first big test for Harris.

Now, she and Walz will face their first ground game hurdles as they barnstorm Pennsylvania and then Wisconsin, Michigan, Arizona and Nevada

Pennsylvania is part of the "blue wall" that carried Biden to victory in 2020 -- one reason many had expected Harris to pick Josh Shapiro, the state's governor.

Source: AFP, by Gregory Walton, with Michael Mathes in Washington