Philadelphia, Pennsylvania - 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.

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"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," she continued.

"I'm here today because I've found such a leader – Governor Tim Walz!"

Walz told the packed Philadelphia arena that Republican White House nominee Donald Trump was too selfish to serve Americans.

"Donald Trump – he sees the world differently. He doesn't know the first thing about service because he's too busy serving himself," the VP pick said.

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."

Kamala Harris and Tim Walz will face their first big campaign game hurdles together as they barnstorm Pennsylvania and then Wisconsin, Michigan, Arizona, and Nevada