The concern with someone like McCain would the presence of another Pat Buchanan or Pat Robertson-like figure that might draw the religious right into a miinor party like the Reform Party (or as an Independent) and we'd have a repeat of the '92 election.
I actually think McCain might well win against Hillary even with such a third candidate in the race. (I so want to believe it's not going to be her, but there doesnt seem to be any apparent alternative on the horizon, and any name that might still turn up would have one hell of an uphill battle against her name recognition).
I mean, how much would a Robertson/Buchanan type candidate on the far right get? Perot got 17% but he was working from the center, hauling in independents. A Robertson/Buchanan type wouldnt go anywhere among those, they'd totally have to rely on the rightwing of the Republican Party itself (at most a Buchanan could get a fringe of anti-immigration independents).
Now both Buchanan and Robertson got how much, at their highpoint, in the Republican primaries? 10%, 20%? Part of whom are loyal Republicans who'd never veer from the party - just see how badly Buchanan did in '04. But imagine, nevertheless, that 10% of Republicans would vote for such a guy - then thats still only 3,5% of the overall votes. McCain would IMO easily get more than a 3,5% lead over Hillary, so he'd still be safe (not sure about Giuliani).
Moreover, if this would indeed happen, with the Republican far-right splitting apart into a third party (and by definition being riven itself in the process), and the Republican candidate would still win the presidentship, then that of course would pretty much be the dream
scenario for the future of the Republican Party. The twelve-year hold on the presidency would firmly confirm its political primacy, while the party could at the same time liberate itself from its radical fringes and move to a more centre-right position - that would make it even harder to beat the next time.
I'll go even further: it'd be in the best interest of the country too. A reasoned Republican government with a strong Democratic opposition and the far-right torn into the margins would be better for the country as a whole too, than a resurgent Clinton-presidency that has a more embittered and mobilised than ever Republican party still under the influence of the far right readying itself for revenge in '12.