Bernie Sanders can kvetch like the great end-of-days preachers of old. When the white-haired senator stands behind the podium, he hunches, and punctuates his points with the tips of his fingers close together, as if grasping a jelly bean. He still has not lost his gravelly Brooklynese after decades in the backhills of Vermont. Income inequality, jobs, financial security—it’s all going to hell in a hand basket. To merely call Sanders a complainer, however, would ignore the urgency of his message: America really is in trouble.

“All of you know what’s going on in America today!” the Vermont senator said in a speech last week in Washington, D.C., where federal workers were rallying for better pay. “We have millions of working people living in poverty, and 99% of all new income is going to the top 1%. That is not what America is supposed to be about!”

The crowd surrounding…