Here's my argument, and I've said it before:
Divorce a thing from its purpose and you kill the thing.
The purpose of the GOP is to repeal the Great Society, or at least some of it.
By making McCain a Republican President, you will divorce the GOP forever from this purpose. It'll just be another European 'soft-right' party. And that means you'll never get out of the mess. Eventually a Democrat will get elected and institute more socialism. It's inevitable if you don't try and push them back.
But if Obama gets elected - which he likely will - and later we get a real Republican candidate, we can at least start slowly hacking away at the FDR-LBJ-BHO legacy.
If it is potentially possible to undo FDR and LBJ, then we can also undo BHO [which is NOT like LBJ and FDR].
If it is NOT possible to undo them, then it doesn't matter who you vote for, we are screwed anyhow.