...the backup quarterback. The current quarterback is a bum that we can blame for all our losses. The backup will come in an save the day. Of course, once he's actually put in the game, he losses his popularity.

Well, politics is not too dissimilar. As long as Obama is not officially in the race, he will be the darling of the media. The second he announces for P||VP, his relationship with the media will completely change. Personally I'd vote for him, and I really car not whether he has a chance to win. If we're going to let the polls decide the outcome in advance, then we mind as well not bother voting.

As for assassination, I think it likely that any and all presidents are under a constant threat - no matter their race or politics. About the only thing you can do is have someone as VP that no one really wants to take over.