Following the scenario: Following Lincolns election victory, the South actually reads the U.S. constitution for once and realizes, that the Republicans wont be able to actually abolish slavery without the neccessary 4/5 majority of the states ratifying a constitutional amendement. They forget the seccession idea and decide to simply continue on opposing abolitionism through political means.
Then in the following years the Lincoln administrations repeals the fugitive slave law, makes sure slavery cannot spread to any new territories, etc etc. But cannot actually do anything to eliminate Slavery in the South.
Eventually Abolitionists decide that the only way to get rid of Slavery throughout the nation is to add enough free states to have the neccessary 4/5 majority for a constitutional amendement.
So which way do they look to expand to?