Alot of the history I've read about the slavery debate indicates that in there was a belief at the time that if slavery in the US could not expand, it was doomed. Therefore, opposition to the expansion of slavery amounted, in a roundabout way, to abolitionism. Why did people believe that slavery required expansion? And, (hopefully this isn't too much of a counterfactual) to what extent were they right?
At the time, intensive farming of cotton heavily depleted farmland. There was a continual movement westward within the south of both plantations and slaves, and most of the "Deep South" had only been developed for 30-40 years prior to the Civil War. Lands west of Texas weren't considered suitable for cotton planting, but many Southerners (including Southern pro-slavery paper Debow's Review) thought there would be a profitable future in slave-driven mining operations.