Madison: “The right of suffrage was much more diffusive in the Northern than the Southern States; and the latter could have no influence in the election on the score of the Negroes.”
And again: (summary of a speech) "He contended that the States were divided into different interests not by their difference of size, but by other circumstances; the most material of which resulted partly from climate, but principally from the effects of their having or not having slaves. These two causes concurring in forming the great division of interests in the U. States. It did not lie between the large & small States: It lay between the Northern & Southern, and if any defensive power were necessary, it ought to be mutually given to these two interests. He was so strongly impressed with this important truth that he had been casting about in his mind for some expedient that would answer the purpose. The one which had occurred was that instead of proportioning the votes of the States in both branches, to their respective numbers of inhabitants computing the slaves in the ratio 5 to 3, they should be represented in one branch according to the number of free inhabitants only; and in the other according to the whole no. counting the slaves as if free. By this arrangement the southern Scale would have the advantage in one House, and the Northern in the other. "
http://people.uncw.edu/lowery/pls101/wilson_chapter_outlines/The Proslavery Origins of the Electoral College.pdf
http://repository.law.umich.edu/cgi/viewcontent.cgi?article=1110&context=mlr
http://digitalcommons.law.yale.edu/fss_papers/790/
and a recent interview with the author of the third paper:
http://www.vox.com/policy-and-polit...ump-electoral-college-slavery-akhil-reed-amar
please don't shoot me for the Vox link,
@mezzrow