The law of thirds
One third of the populace is commited Right, one third committed Left, the remainder uncommitted and historically willing to go either left or right depending on the candidate. You're making the fundamental mistake of assuming that the Republican Party now owns the half of the uncommitted that voted for Bush in the last election. They'll be in play in the next election too.
Fine with me if you think I'm all wet. The chronic hubris and short-sightedness of the Right is the Left's big advantage.