Painting an antagonist as a moral pollutant carries significant political advantages because a group of people who believe that their deeply held values are under threat will become extremely motivated to defend them. Unless everyone works together to defend against those who deliberately flout common morality, the very principles that define the community are called into question. Once a leader, whether Trump or Hammurabi, successfully persuades their followers that an opponent embodies this pollution, all that’s needed to trigger a hostile reaction is a passing reference to them — seeing a member of the squad speaking, say, or seeing a friend or relative retweeting them (“They’re even getting to my little cousin”). Trump and his team are aware of this: The Washington Post reported that one person close to the campaign said, “the general assumption with everything Squad-related is this helps shore up our base.”

Moreover, immorality contaminates whatever it touches. Psychologists have found that experimental subjects are hesitant to touch, let alone wear, a sweater they were told belonged to a murderer. They also found that this “moral contagion” also extends to a sweater designed but never touched by an immoral person. In the case of the squad, it extends to whomever a member of the squad endorses, to whichever legislation they propose, and to whatever causes they champion. This isn’t, of course, to say that the president’s attacks are in any way warranted, nor that both sides are equivalent in their behavior, but to demonstrate that this rhetoric can be effective regardless of the truth of the underlying accusations.

Polling data illustrates the effect of moral contagion. One classic political example comes from surveys of voters who became less supportive of the Affordable Care Act when it was described by poll takers as Obamacare, even though the description of the policy was the same. In one CNBC poll, those who said they were opposed to the ACA rose from 37 percent to 42 perce