For liberals pondering how to sustain a quarantine in a more populist future, the right’s experience with (and leading up to) Trump offers several lessons. The first is the obvious one: For a quarantine to work, you have to be willing to commit to it even when it has electoral costs. That’s emphatically not what Republicans working to sideline Steve King are doing; they’ve turned on him only after a hard-fought midterm election in which they didn’t want to lose his seat. In this sense the partial G.O.P. abandonment of Roy Moore, which helped cost the party a Senate seat, was a more limited but also more important case study in how to establish a cordon sanitaire.

The ascent of Trump was the opposite case. The party establishment could have worked to marginalize or exclude him from the primary process; the birtherism alone offered grounds enough. But the G.O.P. feared the possibility of a Trump third-party run too much to take a stand or draw a line — and because it didn’t, imagining that he could never win, many more lines have subsequently ended up erased.

The second lesson is less high-minded and more complicated. You can’t make your quarantine too broad, or you’ll end up repressing ideas that need to be debated, and empowering demagogues when they’re the only ones who will talk about them.