America’s introduction to Gov. Tim Walz of Minnesota began with a half-hour of cheering for Vice President Kamala Harris and ended with some of the sharpest attacks Democrats have leveled against former President Donald J. Trump and his running mate, Senator JD Vance of Ohio.

Before a raucous crowd in Philadelphia, Ms. Harris and Mr. Walz presented the Minnesotan as a folksy former schoolteacher and football coach who had inspired his students, served in the military and improved his constituents’ lives.

But it was his ability to deliver searing yet accessible attacks against their Republican opponents that won Mr. Walz a place on the national ticket, and during his first rally, he did not miss his marks.

“Donald Trump sees the world a little differently than us,” he said. “First of all, he doesn’t know the first thing about service. He doesn’t have time for it because he’s too busy serving himself.”

Then came the dagger. “Violent crime was up under Donald Trump,” he said, before pausing for effect. “That’s not even counting the crimes he committed.”

The Harris campaign’s presentation of Mr. Walz brought an end to its two-week sprint to find a running mate, a process that typically takes months and involves a series of public tryouts. Instead, Ms. Harris’s search for a partner took place largely behind closed doors.