Mitt Romney took arguably the harshest line on immigration during the Republican Primary. A candidate who is against the DREAM act, thinks undocumented immigrants should self-deport, supports Arizona's immigration law and says "I'm running for office for pete's sake, I can't have illegals" is going to fail badly with the Latino vote in 2012. I submit the ship has already sailed and Romney's ability to attract Latino votes is irreparably damaged. It's not like he had appeal there (or with any minorities) to begin with.
Is he going to reverse position in the general election? That would be something we have seen many, many times before.
I somehow doubt that the GOP base will allow him to. Especially on an issue with racial overtones as this.