If the Toilet of Doom had offed Bush back before the primary season, McCain would have had a good shot at the nomination. But given that now the nominee would be chosen by the convention delegates, all of whom are pro-Bush, the likelihood that they’d choose McCain is infinitesimal.
I think they’d find Guiliani a bit too moderate for their tastes as well, but they might just not take a close enough look to realize that. Still, he’s never held an office higher than mayor. And even if we’re talking about the mayor of NYC, that’s still a ways down in the rankings, so to speak.
Your candidate pool really starts with the Republican governors and US senators, and toss in Cheney and Powell for good measure. Then filter by: (a) will they be sufficiently conservative to satisfy the delegates? and (b) will they be perceived by that group as having sufficient appeal and experience to have a reasonable chance of being elected?
I come up with two names that I think would best survive that filtering: Jeb Bush and George Allen. Jeb has been governor of FL since January 1999; Allen was governor of VA from 1994-98 and has been senator from VA since January 2001. Given that Jeb’s not only GWB’s brother, but is far better known nationally, I’d bet my money on Jeb. Plus he represents a much bigger state (and a swing state at that).
IMHO, if anything serious were to happen to Jeb, be it the Toilet of Doom, major scandal, or doing such a lousy job as guv that even Pubbies didn’t want to be seen with him down there, I think Allen would have the inside track for the GOP Presidential nomination in 2008 if he decided to run.