Well, it depends.  If present trends continue, and 2008 is another sweep for the Dems, at some point the Republicans will have to face up to the reality that their platform just isn't appealing any more.  At that point, they'll have two choices: (1) reinvent themselves, or (2) wait for the ruling party to screw up so they can win by default.  (You'll recognize #2 as the current Democratic strategy.)

If a Democratic President doesn't get us disentangled from Iraq before 2012, the Republicans will most likely find a candidate who can run to the left on that issue.  If we pull out and things don't go well, maybe we could expect a firebreather who will claim vindication for Bush's policies at long last.  On the other hand, if we elect someone who does a basically good job, they've left searching for a Bob Dole-type to be their sacrificial lamb.  A lot depends on events, since they obviously don't have any clear successor in the wings.