When the Republican party offers up crap as a candidate people WILL go elsewhere. Unfortunately the Republican Party, much like the Democrat party, will never acknowledge their mistakes.
My point is that the Dems hold their ranks in presidential elections. They always have. Republicans run-off in all directions over lack of purity (it goes all the way back before Teddy Roosevelt's Bull Moose temper tantrum saddled America with Woodrow Wilson).
It looks like it is going to happen again in `08. Rather than getting 60%, 70% or 80% of what they want, the purists want 100% or they'll split-off enough of the vote to elect the Dem, thereby assuring they get 0% but will have "taught the party a lesson!"