Minneapolis’s Metrodome hummed, its sidelines jammed on a fall Friday night. The Mankato West Scarlets, who had started the 1999 high school football season with a miserable record of 2-4, had improbably swaggered into a state championship game. Now, less than three minutes stood between them and a title.

Their defense, though, would first have to repel the Cambridge-Isanti Bluejackets, who were from north of Minneapolis and trailed by a single touchdown. The quarterback hurled a desperate pass toward the end zone. But inside the 10-yard line, a Scarlet defender intercepted the ball, effectively clinching a 35-28 victory and Mankato West’s first championship.

A coach lifted Tim Walz, then the defensive coordinator and now the Democratic candidate for vice president, skyward. In Mr. Walz’s telling, the victory also eventually proved a launchpad for politics.

Vice President Kamala Harris and her campaign are touting Mr. Walz’s coaching to broaden the Democratic ticket’s appeal, a time-tested strategy that Mr. Walz himself has long used.

His time as a coach serves as a glimpse into the personality he is bringing to the playing field of politics, with friendliness and warmth masking a contagious competitiveness. And Mr. Walz’s years in coaching show a ruthless willingness to change tactics as necessary.