This election is VERY winnable for Christie if he's only down 1 against Clinton right now.
But this election won't happen in a vacuum.
Christie's going to have to get the nomination first, and to do that he will have to say some things to make himself more palatable to the far right wing of the party. Clinton, if she runs, is probably set for the nomination this time.
Christie is also going to have to deal with people at campaign stops heckling him or asking him intrusive questions, and he's going to have to be less blusterous in his responses, lest he make a news cycle-dominating gaffe/controversial remark.
And I find it hard to believe Hilary isn't that "cool" among young voters. She was a meme!
