WASHINGTON: US Vice-President Kamala Harris introduced her running mate Tim Walz, the Governor of Minnesota, and herself to a Pennsylvania stadium packed with ebullient supporters as "middle-class kids" whose goal will be to strengthen the middle class if elected.

Walz stepped right into his role of an "attack dog" as the vice-presidential nominee by building on a catchphrase he had already given the Harris campaign.

He added the adjective "creepy" to "weird", a word he used earlier to describe the rival Republican campaign of former President Donald Trump and his running mate JD Vance.

"Since the day that I announced my candidacy, I set out to find a partner who can help build this brighter future and 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. Promise of freedom, opportunity and justice not just for some but for all. So Pennsylvania, I'm here today because I found such a lever. To those who know him best," Harris said.

"We love our country. And I believe it is the highest form of patriotism to fight for the ideals of our country," Harris said, "That is how we preserve the promise of America. And after all, you know the promise of America is what makes it possible for two middle-class kids -- one, a daughter of Oakland, California, who was raised by a working mother: the other a son of the Nebraska plains who grew up working on a farm -- possible for them together to make it all the way to the White House?"

Harris's candidacy has galvanised the Democratic Party, injecting a strong dose of energy that is drawing comparisons with the 2008 campaign of Barack Obama, who went on to become the first African-American President.