If productive argument is a give-and-take that ends with some kind of meeting of the minds, arguing about transgression of moral lines is more like zero-sum warfare, a battle for supremacy, in which territory is conquered and enemies are banished. The point is less to convince your opponent that she has made an error of reasoning or is wrong on the facts as to convince your own side, as well as the dwindling crowd of ostensibly neutral observers on the sidelines, that they are excused from having to take your opponent seriously because she has crossed a line beyond which people shouldn’t be granted a hearing.

This dynamic plays out more and more in our public arguments. The fights Rep. Ilhan Omar (D-Minn.) keeps igniting over Israel and Jewish influence in Washington take this form every time. Omar tends not to criticize specific policies of the U.S. or Israel on the merits, calling them contrary to the American national interest or needlessly cruel to Palestinians. Instead, she claims that those on the other side of those questions wish to silence her and that they are motivated to take their positions out of dual loyalty or the malign influence of well-heeled lobbyists working to advance Israel’s interests in Washington.