I think a big issue in the whole issue of the expansion of slavery was that the slaves were reproducing so rapidly that some states main export was slaves. Forward thinking white southerners realized they would soon be drastically outnumbered by Negro and Mulatto slaves. Haiti's successful slave rebellion with its attendant horrors weighed heavily on their minds. If the slave states could be cordoned off and not allowed to expand the southerners realized they were the next Haiti and could not only expect no help from the national government but might instead likely get steady agitation of their slave population from the northern abolitionists. The import of Negro African slaves is the worst thing ever done to the U.S. but by 1776 the situation was well entrenched.
The Southern States wouldn't have been able to export any significant number of slaves to the newly entering states because they didn't have the kind of economy that could use them in any large numbers. The whole issue with maintaining a balance between "Slave States" and non "Slave States" had to do with faction blocks in the Federal Government. The South was aware that the North had interests in conflict with their own, and sought to dominate and exploit the Southern States, and this conflict over whether the new states would be free or slave had to do with that alone.
