Winning the Electoral College while losing the popular vote

Discussions have surfaced again over the possibility of a President being elected without a popular vote majority.  The relationship between the winning candidate’s Electoral Vote margin and his popular vote margin in elections since 1960 looks like this:

We can use the estimated equation, 3.78*Popular_Vote-1.33 to solve for the popular vote proportion that predicts an Electoral College tie of 0.5.  That value is .484, meaning that the winner’s share of the two-party vote could be as low as 48.4% and still result in an Electoral College victory.  (Excluding the 1980 outlier from the analysis only changes this value to 48.6%.)

Nate Silver’s 538 blog gives probabilities of 4.8% for a Romney popular vote victory but a loss in the Electoral College; for Obama, the probability is 2.5%.  At 7.3% this is still a chance event, but one whose chances of occurrence may be increasing.