The Most Hotly Debated Issue in the Forming of the U.S. Constitution

Contrary to popular belief it was not slavery.  Although slavery was probably the second most debated issue in our framers' drafting.  Before the Convention delegates moved on to sectional North vs. South debates in the summer of 1787, they had to figure out a way to balance power between large and small states.

The small states, Connecticut, New York, New Jersey, Delaware, and Maryland wanted each state to have an equal say in the soon to be created congress as had been the case from the end of the Revolutionary war for eleven years.  While Massachusetts, Pennsylvania, Virginia, North Carolina, South Carolina and Georgia insisted on representation proportional to population.

Roger Sherman of Connecticut suggested a middle ground between the large and small states.  On Monday June 11, 1787, he proposed:

that the proportion of suffrage in the first branch should be according to the respective numbers of free inhabitants; and that in the second branch or Senate, each state should have one vote and no more.... As the state would remain possessed of certain individual rights, each state ought to be able to protect itself; otherwise a few large states will rule the rest....

Sherman's proposal differed only slightly from what would later be called the Connecticut Compromise and result in our current congressional makeup.

Source: Richard Beeman's "Plain Honest Men." Random House 2010 paperback edition pages 150 & 156.