I was going ask this question at greater length but I see that Cato's Daniel Griswold beat me to it -- if anger over immigration is such a big Republican issue this year, why are the single-issue anti-immigration candidates dropping like flies, while the restrictionist-despised Huckabee and Sen. McAmnesty himself do the lambada on Varmint Mitt's head?
Maybe it's just that border activists can claim satisfaction in helping push the originally soft-on-Mexicans GOP field toward Tancredo.... At any rate, I've yet to see much evidence that they keep coming amounts to winning primary politics, let alone a general-election ticket to anything but failure. (Please leave counter-examples in the comments, etc.)
Here's something bizarre to think about -- if Romney gets a fatal crippling in New Hampshire, and Thompson/Hunter fold up their small tents, the biggest control-the-borders Republican left standing in the race might be ... Ron Paul?
Dave Weigel wrote a year ago about why anti-immigration conservatives fell flat in 2006. More recently Steve Chapman explained why the GOP shifted right on immigration.