Today, Labor Day is largely an occasion for sales, end-of-summer cookouts and back-to-school preparations. Why? Because the movement has become as irrelevant to most Americans as the medieval guilds that preceded it — and all too often a protector of privileges rather than a force for the oppressed. In 1954, more than one in three American workers was a union member. Today, it’s barely over 6 percent of private-sector workers — but, in a huge shift, more than a third of public-sector workers (nearly 35 percent). Indeed, more than half of all union members today work for government: 7.2 million,...