"Instead of open minds and lively debate, I found dogmatic progressive ideology and groupthink. One attendee told me that I, a former Hillary Clinton campaign staffer, am “no better than Roger Ailes” because my company aggregates both liberal and conservative commentaries and thereby is “pushing a right-wing agenda.” Someone else said: “Trump supporters are so stupid . . . they think the tax bill was a good deal because they got back—what, a few thousand a year?” I don’t claim to have the answers. I am, after all, a card-carrying member of the liberal elite. I went to high school...