If John McCain is the nominee, will immigration be off the table?
Short answer: Not at all.
Long answer: If the Republican candidate is John McCain, will the GOP be unable or unwilling to attack the Democratic candidate (whether Barack Obama or Hillary Clinton) on their immigration positions?
The answer clearly is yes: the GOP leadership - just like the Democratic leadership - supports illegal immigration. Certainly, some 527 groups which are to some extent only nominally unaffiliated with the GOP may decide to press the issue, but the parties and the candidates won't. The mainstream media will likewise do what it does now: support illegal immigration in whatever way necessary.
So, if those were all the actors involved, then those who support illegal immigration could breathe a sigh of relief.
Unfortunately for those supporters, there will always be people who aren't partisan hacks and who won't fall into line behind either candidate. And, there are millions of Ron Paul voters who'll be hoping for him to run as an independent. There are probably hundreds of thousands or millions of people who'd like Lou Dobbs to run. And, there are hundreds of thousands of people who are in one way or another actively opposed to illegal immigration.
If the Ron Paul supporters and others they take my advice and go ask the candidates tough questions - the kind the MSM is too corrupt to ask - and then upload their responses, a couple hundred thousand Youtube views later the landscape will change dramatically. Note that the MSM will be completely disintermediated, and there's nothing they'll be able to do.
Ron Paul supporters and others could ask Obama about his indefensible support for illegal immigration, and likewise with Hillary Clinton and John McCain. Just because the MSM is unwilling to press them on those points doesn't mean that it can't be used to show they aren't qualified.
A regular citizen has already asked McCain about the last issue, but the question wasn't as good as mine and didn't mention the most salient point. A better question and a questioner who's familiar with these issues in greater depth will have a far greater impact.