Mind you, I have no proof of any of this. But it seems to me to be much easier to give a candidate a chance at the nomination (as Iowa and New Hampshire have done for Huckabee and McCain, respectively) than it is to ratify that decision (as McCain is hoping Michigan will do).
... As such, voters who are experiencing anxiety will have an easier time pulling the lever for Romney. Voting for Romney only means he stays in the game. But voting for McCain means he has a strong chance of winning the nomination.