From our interview with Nate Silver
One thing people maybe don't grasp as a very basic thing is the more time runs off the clock, the more holding a narrow advantage is likely to hold up. If you were making an assessment of what Obama's chances were in the spring, then you have to worry about a blowup in Europe, you have to worry about something happening in the Middle East -- maybe it still will in the Middle East. Maybe instead of having merely mediocre jobs numbers, actually going back into a recession. There are all these shoes that can drop on you and they haven't.