WASHINGTON >> Democratic presidential nominee Kamala Harris and her newly selected vice presidential running mate, Minnesota Governor Tim Walz, campaigned for the first time together today in Philadelphia, kicking off a multi-day swing state tour aimed at introducing Walz to the national stage.

Speaking to a raucous crowd of more than 10,000 at Temple University, Harris described Walz’s background as a former high school teacher and football coach, an Army National Guard veteran and a former congressman, before predicting that he would earn a new title in November: 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.

>> RELATED: Reactions to Kamala Harris’ pick of Tim Walz for vice president

Vice President Harris 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.

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, former President Donald Trump and his running mate Senator JD Vance.

Walz was elected to a Republican-leaning district in the U.S. 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.

“As a governor, a coach, a teacher and a veteran, he’s delivered for working families like his own,” Harris said in a statement released by her campaign.

Trump and Vance were quick to criticize 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 in Philadelphia earlier in the day.

Americans typically focus on the person at the top of the ticket when choosing whom to vote for, but vice presidential candidates can help or hurt their running mates based on their backgrounds, home state popularity and ability to sway important constituencies or independent voters.

“She went with her gut on this one and chose the option that won’t alienate young folks,” said Republican strategist Rina Shah.

Walz beat out Pennsylvania’s popular governor, Josh Shapiro, for the No. 2 role. Shapiro had faced sharp criticism from the left, especially progressive groups and pro-Palestinian activists, over his support for Israel and his handling of college protests sparked by the war in Gaza.

Shapiro delivered a fiery speech at the Tuesday evening rally in his home state, attacking Republicans and promising to “work my tail off” to get Harris elected. He also offered a strong endorsement of Walz, telling the crowd that he is an “outstanding governor” and a “great patriot.”

Some Trump advisers were glad Harris did not pick Shapiro because of concerns he could help deliver all-important Pennsylvania if he was on the ticket, one adviser said.

Harris and Walz will seek to build on the momentum that the vice president has sustained since becoming the Democratic Party’s standard bearer after President Joe Biden ended his re-election campaign last month. Harris has raised hundreds of millions of dollars and reshaped the race against Trump.

The White House said Harris and Biden spoke before she announced her selection of Walz. The president also spoke to the Minnesota governor.

After their joint appearance in Philadelphia, Harris and Walz plan a multi-city tour of critical swing states including Wisconsin, Arizona and Nevada. Vance is doing a similar tour, with stops in Michigan and Wisconsin planned on Wednesday.

Additional reporting by Andrea Shalal, Jarrett Renshaw, Steve Holland, Gram Slattery and Kat Stafford..