When Donald Trump on Monday chose Senator J.D. Vance of Ohio as his running mate it excited populists — and unnerved some business elites. Later that evening, the president of the Teamsters, Sean O’Brien, gave a prime-time speech at the Republican National Convention. “Over the last 40 years, the Republican Party has rarely pursued strong relationships with organized labor,” O’Brien said. “There are some in the party who stand in active opposition to labor unions — this too must change,” he...