I think we can draw from Bush's poor but improving poll numbers, that a fatigue has set in towards him that makes 2008 a perilous year for the GOP. Primarily it's more Iraq war fatigue, but like G.W.'s father was sunk by the "no new taxes" pledge, I think some of G.W.'s problem in the "no WMD" problem lingers around him, however unfairly.
The peril is that whoever follows Bush in the GOP, may inherit the "Iraq war fatigue" making it impossible for them to get elected.
There is one candidate in the GOP who is innoculated from that fatigue and would, in my opinion, win easily if nominated.
No, he's not a movement conservative, but neither was Nixon, Ford, or G.H.W. Bush. However, I'd take anyone of those 3 without reservation over Hillary or whoever the Dems nominate.
Until and unless a conservative who transcends party label comes along, like Reagan did in spades, and G.W. to a lesser extent, pragmatism must prevail in 2008.
I doubt Scalia or Kennedy will pull a John Paul Stevens and wait until they're 90 to retire.