They all tend to see immigration problems as purely involving illegal immigration, though—and are completely cowardly when it comes to going up against corporate H-1b employers.
We've seen GOP candidates like Tancredo and Buchanan who were trying to be realistic on immigration issues win a few primaries in recent decades.
However, we haven't seen this kind of tough talk coming out of candidates with this kind of serious chance at getting a major party nomination. I don't think any of the current crop have put the kind of serious thought into immigration policy that Tancredo and Buchanan did-but what it shows is the immigration issue is starting to go mainstream. Sooner or later GOP candidates will simply have to start to really address immigration on the campaign trail—and will face serious problems if they don't deliver something real once elected. Immigration restriction is literally the only popular issue the GOP has-and in a troubled economy, it is an issue they must focus on to avoid electoral humiliation.