Quoting the once and future Hindrocket:

The AP says the McCain camp “hopes the announcement of his running mate will stunt any momentum that Democratic rival Barack Obama might get from the just concluded Democratic National Convention.” If it really is Palin, I’m afraid the opposite will happen. Press reaction will be 100% negative; the emphasis will all be on Palin’s inexperience–she’s been Governor of Alaska for less than two years–and the fallout will augment, not limit, Obama’s convention bounce. The most important thing McCain has going for him in this race is the perception that he is the serious candidate. Choosing a running mate who will be widely perceived as unqualified would go a long way toward squandering that advantage.