Lance Banning mines the records of the Constitutional Convention for all sorts of interesting tidbits. Did you know, for example, that James Madison told the convention that (in Banning's words) "[h]e had even been considering a rule that would assure the North and the South that each would dominate one house of Congress"?
Madison's comments on the subject came on Saturday June 30, 1787, during a speech in which he opposed the demand of the smaller states for protection against the larger states. There was no reason to think, Madison argued, that the small and large states had different interests that created a "danger of attack" against the former. "[T]he great division of interests" lay, rather, between the northern and southern states:
Wherever there is danger of attack there ought be given a constitutional power of defence. But [Madison] 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 concurred 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 ["as a security agst. the encroachments of each other" is crossed out].
Madison had therefore considered proposing that the south "have the advantage in one House, and the [north] in the other":
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 of 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. He had been restrained from proposing this expedient by two considerations; one was his unwillingness to urge any diversity of interests on an occasion when it is but too apt to arise of itself - the other was the inequality of powers that must be vested in the two branches, and which wd. destroy the equilibrium of interests.