Don’t interrupt your enemy when he is making a mistake, said Napoleon. So what is President Trump playing at? The further Left the Democrats go, the more determinedly he drags the debate back to the Identitarian culture wars that disfigure modern American politics. When the Democrats are talking about whether men have the right to an abortion, why switch the argument back to whether second-generation Americans should go “back” to “their” countries? One answer might be that, distasteful as they are, such statements are popular. The consensus of the pollsters is that Trump enjoyed a two- or three-point lift after his controversial remarks, though it is necessarily difficult to disentangle the effect of what he is saying from the general craziness of the other side. Again and again, commentators have declared some remark of Trump’s to be political suicide, only for him to rise in the polls. The mocking of John McCain, the attacks on the family of a fallen serviceman, the lurid conspiracy theory about Ted Cruz’s father, the physical mockery of female opponents, all were supposed to sink him. All, instead, floated him higher. Why? One of the more convincing takes on current politics comes from two professors at the University...