Washington DC - Kamala Harris picked Tim Walz as her running mate Tuesday, opting for the Minnesota governor as the partner most likely to complement her in a historic bid for the White House.

Walz had been on a 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 to hold the highest office, 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.

The choice was first reported by CNN early Tuesday morning.

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Expectations had always been that Harris would pick a white man to balance the ticket, and the kind of Democrat who can help counter-attacks from Republicans that she is too far to the left.

Walz fits that description as a 60-year-old Midwesterner with a folksy manner from a state that could be light years from the coastal elites of California, where Harris comes from, or the East Coast.

He will also appeal to progressives after having championed popular Democratic policies, including cannabis legalization, increasing worker protections, and guaranteeing gender-affirming care.

The duo will hit the campaign trail immediately, launching an intense, five-day swing through battleground states starting Tuesday in the biggest prize, Pennsylvania.

Fresh from officially securing the necessary number of delegates for the Democratic nomination overnight, Harris can now head to the national convention in Chicago in two weeks in total control of her party.

It has been a remarkable journey for Harris, who only entered the race last month when President Joe Biden withdrew, bowing to mounting concerns over his mental acuity and ability at 81 to serve a second term.

In a campaign that is barely two weeks old, the 59-year-old former prosecutor has obliterated fundraising records, attracted huge crowds, and dominated social media on her way to erasing what had been Trump's growing lead in polls over Biden.

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The latest presidential poll by the University of Massachusetts Amherst released Monday has Harris leading Trump nationally by three points – 46% to 43% – compared to a four-point lead for Trump over Biden in January.

In the swing states that decide the Electoral College contest in US elections, Harris is neck and neck with Trump.

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

"It tells you about her thought process," Amy Walter, a polling expert from Cook Political Report newsletter, told CBS News.

Now, Harris and Walz will face the first test of their ground game as they make the nationwide swing this week from Philadelphia to Wisconsin, Michigan, Arizona and Nevada. Tropical Storm Debby has forced the postponement of a stop in another swing state, Georgia, and according to media reports also North Carolina.

Pennsylvania is part of the "blue wall" that carried Biden to the White House in 2020, alongside Michigan and Wisconsin. That was one of the main reasons many expected Harris to instead pick that state's governor, Josh Shapiro.

Also on the vice presidential shortlist had been former astronaut and current Senator Mark Kelly of Arizona and Kentucky Governor Andy Beshear.