2/ Madison’s language was as elegant as the truth was harsh: national popular elections were not going to happen because they would not give the slave states as much power as they sought in selecting presidents. White Southerners wanted the electoral strength of their states to reflect their slave populations—which would not have been possible with direct elections, since slaves obviously would never be given the vote. It was, however, possible in an electoral system that linked a state’s voting strength to its representation in Congress—since slaves counted as three fifths of a person in apportioning seats to the House of Representatives.>>

http://www.nybooks.com/articles/2005/03/24/the-electoral-college-flunks/