I think it's true that immigration isn't all that important an issue even to Republican voters, but that doesn't mean that your position on immigration isn't incredibly important. Hating the other is very popular among Republicans even if immigration per se isn't so important. So Republican primary candidates have to signal they've got a sufficient amount of hate, and they do that by wanting to "double Guantanamo," invade several more countries filled with scary Brown people, enacting punitive measures against immigrants, etc.
Immigration isn't the issue. Hating immigrants is the issue.
