Former President Donald J. Trump has chosen Senator J.D. Vance of Ohio to be his running mate, wagering that the young senator will bring fresh energy to the Republican ticket and ensure that the movement Mr. Trump began nearly a decade ago can live on after him.

Mr. Vance, 39, is a political newcomer who entered the Senate only last year, but he has spent that time methodically ascending the conservative firmament. Once an acerbic Trump critic — attacking Mr. Trump as “reprehensible” and calling him “cultural heroin” — he won Mr. Trump’s backing in his 2022 Senate race by wholly embracing his politics and his thinking about a stolen election. The endorsement lifted him above a crowded field, and ultimately to the Senate.