Radio then, like Twitter today, was the technology of the id; a channel that could concentrate political fury at a time when there was plenty to go around.

It was also a time when ideology dictated that fury be directed at entire classes of people. The decade began with Soviet propaganda cheering Stalin’s announcement of “the liquidation of kulaks as a class” — a reference to millions of Ukrainian peasants who would die of forced starvation in the Holodomor.

The political mind-set that turned human beings into categories, classes and races also turned them into rodents, insects and garbage. “Anti-Semitism is exactly the same as delousing,” Heinrich Himmler would claim in 1943. “Getting rid of lice is not a matter of ideology. It is a matter of cleanliness.” Watching Warsaw’s Jewish ghetto burn that year, a Polish anti-Semite was overheard saying: “The bedbugs are on fire. The Germans are doing a great job.”

Today, the rhetoric of infestation is back. In the U.S., Trump uses it to describe Latin American migrants. In Europe, Jaroslaw Kaczynski, chairman of Poland’s ruling Law and Justice Party, warned in 2015 that migrants carried “all sorts of parasites and protozoa,” which, “while not dangerous in the organisms of these people, could be dangerous here.”

More of this talk will surely follow, and not just from the right. The American left has become especially promiscuous when it comes to speaking pejoratively about entire categories of disfavored people.

None of this would be possible without the third factor: the conviction that an opponent embodies an irredeemable evil, and that his destruction is therefore an act of indubitable good. That spirit of certitude that dominated the politics of the 1930s is not so distant from us today. The unpopular political figures of our day are the people who seem to convey less than 100 percent true belief: the moderate conservative, the skeptical liberal, the centrist wobbler.