‘History repeats itself, first as tragedy, then as farce.” I can’t help but mumble this famous sentence from Karl Marx’s “The Eighteenth Brumaire of Louis Bonaparte” whenever I hear about the socialist wing of the new—and distinctly not your father’s—Democratic Party. Socialism caused the deaths of more than 100 million people under Lenin, Stalin and Mao, but young progressives want to give it another go.

Not all the socialist Democrats are young. Sen. Bernie Sanders, 77, proudly calls himself a democratic socialist, but one...