While I have no doubt that McCain would have substantial crossover appeal to the mushy middle crowd, McCain's name doesn't just magically appear on the ballot. How many months of "Gee, Senator McCain, do you disagree with your fellow Republicans on this issue? On this issue? On this issue? On this issue?" do people want to endure?
McCain is a Republican who cannot go out there and campaign against his fellow Republicans.
This is just stupid. A McCain VP would ensure a loss across the board.
