Kamala Harris said on Tuesday she was "proud" to tap Tim Walz as her running mate, opting for the Minnesota governor with blue-collar credentials as the partner most likely to boost her historic White House bid.

Walz had been on a vice-presidential shortlist with a string of other Democratic figures seen as broadening Harris's appeal as she sprints into the contest against Donald Trump.

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.

"I am proud to announce that I've asked Tim Walz to be my running mate," she posted on X.

"As a governor, a coach, a teacher, and a veteran, he's delivered for working families like his. It's great to have him on the team."

President Joe Biden quickly expressed support for his deputy's "great decision" to choose Walz, saying the duo "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 win over 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 "nonsense" 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 seat in the House of Representatives in 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.

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

Democratic grandee Nancy Pelosi swatted such criticism aside, telling MSNBC that Walz "is right down the middle. He's a heartland of America Democrat."

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