Vice President Kamala Harris, the Democratic nominee for president, has chosen Gov. Tim Walz of Minnesota as her running mate, elevating a former football coach whose rural roots, liberal policies and buzzy takedowns of former President Donald J. Trump have recently put him on the map.

Mr. Walz, 60, emerged from a field of candidates who had better name recognition and more politically advantageous home states. Minnesota is not a top-tier presidential battleground and is unlikely to prove critical to a Harris-Walz victory.

But he jumped to the top of Ms. Harris’s list in a matter of days, helped by cable news appearances in which he declared that Republicans were “weird.” The new, clear articulation of why voters should reject Mr. Trump caught on fast and turned the spotlight on the plain-spoken Midwesterner behind it.

“One of the things that stood out to me about Tim is how his convictions on fighting for middle class families run deep,” Ms. Harris said in a social media post confirming his selection. “It’s personal.”

Mr. Walz will appear with Ms. Harris at a rally on Tuesday evening in Philadelphia, their first appearance as the Democratic ticket.

Mr. Walz leapfrogged better-known contenders in part because Ms. Harris viewed him as an Everyman figure from Minnesota whose Midwestern-dad-vibe balanced out her Bay Area background, according to three people familiar with the vice president’s thinking.