Democratic presidential candidate Kamala Harris and her newly chosen running mate, Tim Walz, are scheduled to hold a rally in Philadelphia on Tuesday evening.

Harris, the U.S. vice-president to President Joe Biden who became the Democratic nominee after Biden dropped out of the race, announced earlier on Tuesday that she had picked Walz and said in a post on social media that he has "delivered for working families" as a governor of Minnesota, coach, teacher and veteran.

Walz called it "the honour of a lifetime" to be Harris's vice presidential pick.

In choosing the 60-year-old Walz, Harris is elevating a Midwestern governor, military veteran and union supporter who helped enact an ambitious Democratic agenda for his state, including sweeping protections for abortion rights and generous aid to families.

It was her biggest decision yet as the Democratic nominee, and she went with a broadly palatable choice — someone who deflects dark and foreboding rhetoric from Republicans with a lighter touch, a strategy that the campaign has been increasingly turning to since Harris took over the top spot.

Walz is joining Harris on the ticket during one of the most turbulent periods in modern American politics. Republicans have rallied around former president Donald Trump after he was targeted in an attempted assassination in July. Just days later, Biden ended his re-election campaign, forcing Harris to scramble to unify Democrats and decide on a running mate over a breakneck two-week stretch.

Following the rally, Harris and Walz will spend the next five days touring critical battleground states, visiting Eau Claire, Wis., and Detroit on Wednesday and Phoenix and Las Vegas later in the week.