In his remarks to a raucous crowd of more than 10,000 at Temple University on Tuesday, the Minnesota governor described his upbringing in a small Nebraska town, his 24 years serving in the Army National Guard and his prior career as a high school social studies teacher and football coach.
"It was my students who encouraged me to run for office," he said.
"They saw in me what I was hoping to instill in them: a commitment of common good, a belief that one person can make a difference."
Tim Walz says his students encouraged him to throw his hat in and run for office. (AP PHOTO)
He also went after the Republican presidential nominee, Donald Trump, and his running mate, Senator JD Vance, an early demonstration of how Walz will approach the traditional "attack dog" role of the vice-presidential candidate despite his affable, folksy style.
"He mocks our laws, he sows chaos and division, and that's to say nothing of his record as president," Walz said of Trump.
"He froze in the face of the COVID crisis, he drove our economy into the ground, and make no mistake, violent crime was up under Donald Trump. That's not even counting the crimes he committed."
Harris' entry into the race after President Joe Biden abandoned his re-election bid just over two weeks ago has rapidly upended the election campaign, with polls showing she has erased Trump's lead.
Walz criticised Republicans for pursuing restrictions on women's reproductive rights, an issue that has plagued Republicans since the US Supreme Court in 2022 ended women's constitutional right to abortion.
"Even if we wouldn't make the same choice for ourselves, there's a golden rule: mind your own damn business!" he said, drawing a huge ovation.
Harris, speaking before Walz, listed his titles - husband, father, teacher, coach, veteran, congressman, governor - before predicting he would earn a new one in the November election: vice-president of the United States.
"He's the kind of person who makes people feel like they belong and then inspires them to dream big," she said.
The Democrats raised more than $A31 million after announcing Walz as the vice-presidential pick. (AP PHOTO)
Harris, the US vice president, announced her choice of Walz earlier in the day, opting for a vice-presidential running mate with executive experience, military service and a track record of winning over the rural, white voters who have gravitated to Trump over the years.
The Harris campaign said it had raised more than $US20 million ($A31 million) after the announcement of Walz as the vice-presidential pick.
Pennsylvania, the site of their first rally, is seen as perhaps the most critical state in what is expected to be a close election between the Democrats and their Republican rivals.
Walz was elected to a Republican-leaning district in the US House of Representatives in 2006 and served 12 years before being elected governor of Minnesota in 2018 and again in 2022.
He has pushed a progressive agenda that includes free school meals, goals for tackling climate change, tax cuts for the middle class and expanded paid leave for workers.
Trump and Vance were quick to criticise the new competition as too liberal.
"This is the most Radical Left duo in American history," Trump wrote on his social media platform.
Vance knocked Walz for his handling of protests after George Floyd, a Black man, was killed in Minneapolis by a white police officer in 2020, with the Republican saying Walz was not assertive enough in combating the rioters.
"The biggest problem with the Tim Walz pick - it's not Tim Walz himself. It's what it says about Kamala Harris, that when given the opportunity she will bend the knee to the most radical elements of her party," Vance told reporters.
Walz beat out Pennsylvania Governor Josh Shapiro, for the number two role. Shapiro had faced sharp criticism from progressive groups and pro-Palestinian activists, over his support for Israel and his handling of college protests sparked by the war in Gaza.
After their joint appearance in Philadelphia, Harris and Walz plan a multi-city tour of critical swing states including Wisconsin, Arizona and Nevada.Â
