If you arrange the states that gave their electoral votes to President Obama in order of how close the race was in each state, and strip away states that he didn't need to win a majority (as opposed to in order of the time of day when a race was called, in which case the prize goes to Ohio), it appears to me that Colorado where Obama won 51.2% to 46.5% (a margin of 4.7 percentage points, quite a bit more than state pollsters had predicted before the election) was the state that pushed President Obama over the 270 electoral vote threshold.
Florida (0.6 percentage points and 29 electoral votes), Ohio (1.9 and 18 electoral votes) and Virginia (3.0 and 13 electoral votes) all had closer races, but in hindsight, President Obama would have still won 272 electoral votes without them. President Obama won the other swing states, New Hampshire (5.7 percentage points), Wisconsin (6.7), Iowa (5.6) and Nevada (6.6), by wider margins than he did in Colorado.
The wide margin for President Obama in the marginal state makes a repeat of the 2000 election which was decided by the U.S. Supreme Court, in the case of Bush v. Gore, impossible.
Cross posted at Colorado Pols.