In the course of three years I just spent on the road speaking with voters about President Trump, what surprised me the most was how few people have actually changed their minds about him. With rare exceptions, those I spoke with who supported him on Election Day 2016 still support him today, and the same is true of those who opposed him. That’s not to say that people’s views of the president have not changed. But where they have changed, the change has all been in one direction — toward a more extreme and more deeply entrenched conception of the president. This is the product of the increasingly tribalistic nature of American politics. Tribalism is predicated on group loyalty, but in the political sense, I found it has two other prominent features. The first is...