Actually it would take a rewriting of the Southern Constitution to have slavery abolished in even a state by state basis because of the "in transit" clause. Dred Scott made it clear you could be "in transit" for years.
I just feel that eventually, the majority if white southerners would want to get rid of slavery due to the harm on their economey, and at that point they would try and vote to end it. The key is gettting to this point before a revolution or slave revolt.