The small states would never allow such a change to the constitution. (A presidential voter in Wyoming has almost 4 times the vote of a Californian.) But the big states may have found a work around: http://en.wikipedia.org/wiki/National_Popular_Vote_Interstat...
In short, the states that agree to the compact agree to proportion all of their electoral votes to the winner of the popular vote. (For example, if Romney wins the national popular vote, CA would give Romney all of CA's electoral votes.) The compact will only kick in once the states that have ratified it have 270 electoral votes (enough to win). It's already about halfway there at 132.
How crazy would it be to see campaigning in California and New York?