If he picks someone far left, he can start losing the independents. And, after all, the Democratic base is already energized.

If he picks Clinton, he energizes McCain’s base. And probably doesn’t pick up any significant number of voters that McCain would otherwise have gotten.

Obama might avoid either of those kinds of mistakes. But unless he makes one of those, or some other major gaffe, McCain hasn’t got a real good chance of making up ground on the enthusiasm front.