Minnesota Governor Tim Walz (left) and US Vice President Kamala Harris. (Photo: AFP/GETTY)

In her remarks to a raucous crowd of more than 10,000 at Temple University, Ms. Harris praised the leadership qualities of Mr. Walz, a former member of the US National Guard and a former teacher.

"Since the day I announced my candidacy, I set out to find a partner who can help build a brighter future, a leader who will help 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. So Pennsylvania, I'm here today because I found such a leader – Governor Tim Walz of the great state of Minnesota," said Ms. Harris.

Pennsylvania is a key battleground state in the contest between the Democratic Harris-Walz ticket and the Republican ticket of former US President Donald Trump and JD Vance, who like Walz is a military veteran from the Midwest. After Philadelphia, Harris and Walz plan to campaign in other battleground states, including Wisconsin, Arizona and Nevada. Vance is conducting a similar campaign, with events scheduled on Wednesday in Michigan and Wisconsin.