Once upon a time, abortion wasn’t a defining issue for conservatives and liberals. A candidate could be a pro-choice conservative or a pro-life liberal. Over time that changed. I think we’re in the middle of a similar shift on immigration:

[I]mmigration is rapidly becoming a defining issue for American conservatism.
It didn’t used to be. Ronald Reagan was and is a conservative hero, even though he supported mass immigration and an amnesty. In 1988, Jack Kemp could run for president from the right of the party — with everyone considering him a “movement conservative” rather than an establishment candidate — while supporting open borders. . . .
Obviously, immigration is now a much bigger issue for Republican voters.