Still and all, by the 1850's slavery was pretty much dying out (I can hear you choking on that ). Take a look at what the number one US internal export "crop" for Virginia (and North Carolina I believe) was around 1855, I think you'll be surprised . The particular geography required for cotton was also disappearing as the US moved further west so, unless the US expanded into the Caribbean, you weren't likely to have many more slave states appearing.
I'd like some more backup on the idea that slavery was dying as an economic institution. I'll have to wait until I get home, but most of my texts on this indicate just the opposite.

Also, the aforementioned westward expansion of the U.S. was driven to a large extent by slavery, or specifically the influence of slaveowners interested in maintaining a proslavery majority by carving out more slave states from Mexico and the West. The history of the Texas Revolution and the Mexican War and its backers, as well as the filibuster movement which sought to carve out an empire of American slavery in the Caribbean and Central America, points this out.