Josh Marshall has the best explanation for why Democrats suck at what he calls "bitch-slap politics."

Stuff like the Swift Boat attacks on John Kerry and McCain's Celeb/P Diddy assault on Obama aren't really about the attacks themselves. In themselves, they're often too cartoonish to be believed in any literal sense. What they're about is smacking the other guy around and making him take it. There's no better way to demonstrate someone's lack of toughness or strength than to attack them and show they are either unwilling or unable to defend themselves -- thus the rough slang I used above. That not only makes the other guy look weak. It also transforms him into an object of contempt, which together are politically fatal. It's this meta-message of weakness that resonates far beyond the literal claims. And it's this that Democrats so often seem to miss -- explaining the factual inaccuracies of the claims, demanding that the attacks stop, all the while reinforcing the intended message of the attacks in the first place.