Professor John Cochrane writes in his blog The Grumpy Economist, Why the electoral college is a great idea, 11/4/12
Suppose we had a popular vote instead. Now, instead of fighting for 51% of Ohio, President Obama could instead try to raise his 60% of New York and Illinois to 70%, even if it meant 45% of Ohio. Or he could try to raise his 80% of New York city and Chicago to 90%, (made up number). He doesn’t need to persuade people, really, he just needs to encourage more New Yorkers and Chicagoans to turn out.
Instead of fighting for 51% of Ohio, Governor Romney could raise his 60% of the south to 70%, or raise his 75% of Utah to 85%, or just work to get all those people out to the polls.
A system in which each candidate has to get a small majority in a large number of states is a good system to keep a polarized democracy together. A straight popular vote, in which one could win by getting huge majorities in some areas and lose by huge majorities in other areas, is a disaster waiting to happen. We do this in sports, for similar good reasons: the world series winner is not just the total number of runs in the regular season.
The electoral college puts a lid on extremist policies from both parties that could be dictated from a few heavily populated centers. It affords political stability and better representation.