I agree with earlier posters who said that the biggest threats a Republican nominee faces in 2008 is a conservative third-party candidate who emerges because of dissatisfaction with that nominee.
Re 1968, some say Wallace being in the race was a boost to Humphrey. Nixon even said in his book that Wallace took conservative voters away from him. Nixon’s margin might have been larger. He almost certainly would have carried TX so his electoral college margin would have been larger. Without Wallace in the race Nixon thought he would have taken an outright majority of the popular vote too.