Tens of millions of workers want a union. The most recent national poll, from 2017, found that nearly half of all nonunion workers in the United States would join one if they could. But no one knows for sure what could spur the scale of new organizing needed to meet this demand—so supporters seize on any encouraging signs of the movement’s revival. Labor journalists like to bandy about the public’s approval rating for unions, which Gallup polls in the last few years have put at around 70 percent, the highest rate in almost six decades. But in that same period, the share of unionized private-sector workers has plummeted from 30 percent in 1965 to an abysmal 6 percent today. Unions are historically popular, yet also historically weak. Absent a dramatic turnaround in labor’s favor, the fact is that employers can run still roughshod over 94 percent of workers in the private sector—public opinion be damned. This situation is almost the mirror image of labor’s midcentury peak.…