Normally, with an issue as complex as immigration, it’s hard to pinpoint any one thing that’s driving a notable shift in opinion. Unless there’s a sudden surge at the border, as there was in the summer of 2014 with children from Central America being sent north via Mexico, there’s no obvious reason for public opinion to change dramatically within a narrow six-month window, as it has here. In fact, if you’d asked me to guess whether support for the wall was up or down lately, I’d guess up for the simple reason that Americans are more worried about terrorism...