If I remember correctly Nate Silver called it as around an 80% chance of him getting re-elected, which suggests that statistically it was fairly comfortable? (I'm assuming Silver is pretty reliable on this as he seems to have got things right more often than anyone else in making calls - suggests he knows onions).

Plus the state he got the lowest margin of victory, out of the states he had to win, he got just under 5%, which isn't that uncomfortable. That also leaves him other states in hand he could have won to get the 270.