Kamala Harris was closing in on her vice-presidential pick on Monday, with an announcement expected within 24 hours as she scrambles to introduce herself to the American public with a tour of battleground states just three months out from the election.

All paths to the White House run through a handful of swing states, and Harris will kick off her five-day run Tuesday in the largest, Pennsylvania, as she builds momentum for her showdown with Republican Donald Trump on November 5.

"This election is a fight for our country, our future, and our most fundamental freedoms and rights," she posted on X on Monday.

"We believe in the promise of America – and we're in this fight because we know what's at stake."

Fresh from winning enough delegate votes to secure the Democratic nomination, the country's first female, Black and South Asian vice president heads into the national convention in Chicago in two weeks in total control of her party.

The 59-year-old former prosecutor has obliterated fundraising records, attracted huge crowds and dominated social media on her way to erasing the polling leads Trump had built before President Joe Biden quit the race.

Next on the agenda is a vice presidential pick, with an announcement expected on Tuesday, before her evening rally alongside the mystery nominee in Philadelphia, Pennsylvania's largest city. 

The Keystone State is the most prized real estate among the closely fought battlegrounds that decide the Electoral College system.

It is part of the "blue wall" that carried Biden to the White House in 2020, alongside Michigan and Wisconsin – two states where Harris is due to woo crowds on Wednesday. 

Pennsylvania is governed by 51-year-old Democrat Josh Shapiro, a frontrunner in the so-called "veepstakes" shortlist that also includes fellow state governors Tim Walz and Andy Beshear, as well as Arizona Senator Mark Kelly and U.S. Transportation Secretary Pete Buttigieg.