There are better candidates out there with far more sterling conservative credentials-- Sarah Pallin, Eric Cantor and Tom Coburn, just to name three.
In the end, McCain has to add up the pros and cons. Not just who can help energize the base, but who can put swing states and swing demographics into play. He's also got to pick a team player because whomever he selects will probably give up any chance of ever being president by taking the VP slot. The reason is because if McCain wins and McCain does a good job (neither of which is probable at this point), the VP would have a very steep hill to climb extending Republican administrations to 16 or 20 years. It hasn't happened since the Civil War and it won't happen with amnesty which McCain is sure to push in an even more liberal congress.
Good. Stop right there. Perfect. The biggest political slam-dunk I've ever seen.