In response to a misleading Clinton radio attack (the Clintons? Misleading? Never!), Obama fires back with a South Carolina radio spot of his own. It suggests that the Obama camp may be worried that the Clintons could erode what looks like Obama’s substantial lead in South Carolina polling. The larger problem is, of course, that Obama has been solidly on defense since he lost New Hampshire. He needs a game-changer to win, and a South Carolina victory combined with a strong counterattack – the radio ad says Clinton will “say anything” to become president – may be exactly that.