The GOP lost the Presidency because McCain was the most boring, white bread, non-communicative, same old same old, candidate in the past 50 years.... amnesty or no amnesty.
Voters (even many Democrats) were desperately looking for a viable candidate because deep down they didn't trust Obama.
But when Obama began to outclass and outthink McCain at every turn, even the anti-Obama Democrats threw up their hands and said, 'what the heck.'
People instinctively don't support a 72-year old frumpy guy with recurring cancer, especially one with such an irritating nasal voice as McCain. I truly believe it had very little to do with amnesty one way or another.
Amnesty probably didn't effect those who actually voted, but we have no idea how many conservatives just sat home because really why bother. McLame was just Bob Dole (ver 2.0) and he appealed to nobody. I think the majority of those who voted for him (myself included) were petrified of the alternative, and its IMPOSSIBLE to win an election that way.