On Super Tuesday, two commentators on CNN argued about the Ku Klux Klan. On television. In America. In 2016.

It was a singular moment in cable news. This is partly because, until this week, the K.K.K.’s loathsomeness had seemed to be a settled issue. But also because — as intense and unsettling as the argument was — it was substantive and illuminating in a way time-killing cable shouting matches rarely are.

The exchange began when the conservative commentator S. E. Cupp criticized the Republican front-runner, Donald J. Trump, for the “dog whistle” racial implications of his comments and proposals. (He has, besides denigrating Mexican immigrants and calling for a ban on Muslims entering the country, said that he’s not sure whether he would have opposed internment camps for Japanese-Americans during World War II.)