The Democratic National Committee made the announcement as Vice President Harris appeared alongside Governor Walz after she selected him as her running mate for the November presidential election.

Ms. Harris spoke about the nomination at the campaign rally, emphasizing: "Today, I stand before you to proudly announce that I am now officially the Democratic nominee for president of the United States," according to ABC News.

Hours after being announced as the Democratic vice presidential nominee, Mr. Walz took the stage with Vice President Harris in Pennsylvania, expressing his intention to vigorously push the case against former US President Donald Trump.

"Donald Trump, he sees the world differently. He doesn't know the first thing about service because he's too busy serving himself," Walz told a cheering crowd of about 10,000 supporters in Philadelphia, Pennsylvania, according to AFP.

Mr Walz accused Mr Trump of deliberately undermining the US economy for his own purposes, mocking law and order, and sowing “chaos and division”.

With less than three months to go until the presidential election, many Americans don't know much about Mr. Walz, and he gave voters a glimpse of his personal story, describing his military service, experience as a public school teacher and his political record, according to AFP.

In their first appearance together on the campaign trail, Ms. Harris and Mr. Walz showed how they could appeal to voters, one a black former senator of South Asian descent from California and the other a white former congressman from America’s working-class heartland.

Ms. Harris (60 years old) described Mr. Walz (60 years old) as "a leader who will help unite the country and move us forward, a fighter for the middle class, a patriot who believes in the extraordinary promise of America, like me".

A former attorney general of California, Ms. Harris has sought to draw a contrast between her past as a prosecutor and Mr. Trump’s criminal record, which includes 34 felonies.

But Ms Harris told the crowd: "This campaign - our campaign - is not just a fight against Donald Trump. Our campaign ... is a fight for the future," Ms Harris declared.

Mr Trump and his running mate JD Vance were quick to criticize the new pair as too liberal. “This is the most radical left-wing couple in American history,” Mr Trump wrote on social media, according to Reuters.