“Cultural Marxism” is also a favorite topic on Gab, the social media network where Robert Bowers, the man accused of shooting 11 people at a synagogue in Pittsburgh last month, spent time. Mr. Lind may have only fantasized about mass death as a comeuppance for cultural Marxists, but others have acted on it: In his 1,500-page manifesto, the Norwegian far-rightist Anders Breivik, who killed 77 people in 2011, invoked “cultural Marxism” repeatedly. “It wants to change behavior, thought, even the words we use,” he wrote. “To a significant extent, it already has.”

According to their delirious foes, “cultural Marxists” are an unholy alliance of abortionists, feminists, globalists, homosexuals, intellectuals and socialists who have translated the far left’s old campaign to take away people’s privileges from “class struggle” into “identity politics” and multiculturalism. Before he executes the professors, the protagonist of Mr. Lind’s novel expounds on his theory to their faces: “Classical Marxists, where they obtained power, expropriated the bourgeoisie and gave their property to the state,” he says. “Where you obtained power, you expropriated the rights of white men and gave special privileges to feminists, blacks, gays, and the like.” It is on the basis of this parallel that the novel justifies carnage against the “enemies of Christendom” as an act showing that “Western culture” is “recovering its will.”

Some Marxists, like the Italian philosopher Antonio Gramsci and his intellectual heirs, tried to understand how the class rule they criticized worked through cultural domination. And today, it’s true that on campus and off, many people are directing their ire at the advantages that white males have historically enjoyed. But neither the defense of the workers nor of other disempowered groups was a conspiracy on its own, and never was there a malignant plot to convert the first into the second — which is what “cultural Marxism” implies. Deployed to avoid claims of injustice, the charge functions to whip up agitated frenzy or inspire visions of revenge.

And while increasingly popular worries about cosmopolitan elites and economic globalization can sometimes transcend the most noxious anti-Semitism, talk of cultural Marxism is inseparable from it. The legend of cultural Marxism recycles old anti-Semitic tropes to give those who feel threatened a scapegoat.

A number of the conspiracy theorists tracing the origins of “cultural Marxism” assign outsize significance to the Frankfurt School, an interwar German — and mostly Jewish — intellectual collective of left-wing social theorists and philosophers. Many members of the Frankfurt School fled Nazism and came to the United States, which is where they supposedly uploaded the virus of cultural Marxism to America. These zany stories of the Frankfurt School’s role in fomenting political correctness would be entertaining, except that they echo the baseless allegations of tiny cabals ruling the world that fed the right’s paranoid imagination in prior eras.