The great irony of Bernie Sanders’s socialist revolution is that for it to have any chance of succeeding, the Vermont senator would have to convince his fellow Americans that it wasn’t very revolutionary. In fact, it merely required his adopted political party to return to the path from which it had recently strayed. Democrats, according to Sanders, have broken from the successful formula of President Franklin Roosevelt, who assembled a coalition in part made up of immigrants, city dwellers, left-wing intellectuals, and black people to restructure the economy and the future of the party radically. The New Deal coalition was broader than that, of course, with rural Southerners and trade unionists providing Roosevelt with electoral margins that gave him and the Democratic Party supermajorities for years. But that coalition, to hear Sanders say it, had...