Abortion politics followed a similar trajectory. Opposing abortion wasn’t always considered part and parcel of conservatism. Everybody considered Senator John Tower of Texas a movement conservative even though he supported legal abortion. Over time, though, as conservatives grew more opposed to abortion and liberals more supportive of it, it became an issue that voters used to sort candidates by ideology.

Very few Republican voters have ever told pollsters that abortion is their top priority. But voters who knew that a candidate opposed it could also be reasonably sure that he would oppose gun control and tax increases — or, at least, that he was more likely to oppose it than someone who favored legal abortion. Candidates who favored legal abortion started to have real trouble getting support from a lot of conservative voters. Eventually, such candidates came to be seen as not being conservatives at all.

Today, favoring tighter control of immigration is becoming a stand-in for conservatism in the same way. What that means exactly is a little hard to say. But the same was true in the case of abortion. Could a politician be considered “pro-life,” and thus have that conservative credential, if he favored keeping abortion legal in cases of rape and incest? Over time it became clear that yes, he could. A politician could also meet the test even if he showed no burning passion to fight abortion. If, on the other hand, a politician said it should generally be legal but not taxpayer-funded, he wouldn’t meet the test…

The issue is also becoming a dividing line between the parties in a way it didn’t used to be, because the Democrats are also making it more central to their self-definition. (Again, the same thing happened on abortion: Pro-life Democrats, once common, grew scarce, and then so did Democrats willing to compromise on pro-choice views.) Barbara Jordan was a celebrated Democratic congresswoman; in the 1990s, President Bill Clinton put her in charge of an immigration commission that recommended cutbacks. Nothing similar will happen if there’s another President Clinton next year.