Nearly all American politicians in both major parties support some limits on legal immigration, and some measures to enforce those laws. Virtually all of them define “some human beings as ‘unworthy of legal standing’” — a position Baer insists does not deserve to be defended in public at all. Perfectly cogent arguments can and have been made that, say, Hillary Clinton advocates systemically racist policies or that Bernie Sanders encourages sexism. The ability to associate disagreeable ideas with the oppressor, and to quash free speech or other political rights in the name of justice for the oppressed, is a power without any clear limiting principle. Historically, states that rule on that basis tend to push that power to its farthest possible limit.

The debate has largely centered on campus free-speech battles because academia is one of the few subcultures in American life where the left can wield hegemonic power. But the problem is ideological, not generational, and one can find signs of the phenomenon creeping out into other corners of political life. The illiberal left has used the fear of Donald Trump to goad broader elements of the progressive movement to adopt their repressive methods and slogans. The slogan “shut it down!” has come into fashion on the left. Protesters caused the cancellation of a Trump rally last summer, and were seen chanting “shut it down!” at a conservative think tank this week.

The illiberal left has brought its notion that opposing views can and should be shut down into wide circulation. It has disproportionate influence within the progressive movement, but remains, for now, a noisy minority. But noisy minorities — like the cranks and kooks of the far right who had been, 60 years ago, banished to the margins of Republican politics — have a way of developing over time into majorities, unless they meet forceful opposition.