OK -- we're worried about "moral values" voters. But remember that a big part of what people respond to in the Christian-conservative message is the messenger; energy and personality are central to the tradition.
Now, I don't respond to Bush at all (except with revulsion), but he did a hell of a job of firing up his people -- and "moral values" are a big part of what he used to fire them up.
OK, now think: Of the Republicans who regularly say what Bush says about religion, who's going to fire voters up the same way in 2008? Bill Frist? Er, I don't think so.
I suppose there's always Alan Keyes or Judge Roy Moore, but they appall non-religious voters. Bush is both a full-throated preacher and a "happy warrior," and, much as I hate to admit it, politically that's a rather potent combination (possibly enough to get him to 51% even though as president he's doing a godawful job). I don't think the Republicans are going to find anyone like Bush next time around. And if they don't, a lot of the "moral values" voters really might not vote -- or might vote Democratic.