Ask Lemmygrad

This isn't meant to be "the" answer, just one part of whole.

The major fault line in the antebellum period wasn't allow slavery / abolish slavery, it was whether or not to allow slavery in the "territories" i.e. the western parts of the US that were claimed by the US but not yet considered states. Well, the southern slavers did worry about the wholesale abolition of slavery, but that's because they were always paranoid about that. But for the southern slavers, what happened in the territories was critical because 1.) if more states are created that do not allow slavery, eventually the southern states would be outvoted in the senate and lose their disproportionate power since the senate was created in part as a way to keep the southerners happy. But also 2.) these southern slavers were making huge profits, and those profits needed to be reinvested. These slavers looked at the western territories and had dreams of using their profits to buy land and recreate the plantation system all across the west. Just because slavery was the mode of production, these southern plantations were creating tremendous amounts of capital. Where under capitalism, you might reinvest that capital in better equipment to be more productive with less labor, slavers had no such incentive because their slave labor was so cheap. But you still have to reinvest your capital (the southern economy was, after all, not a pure slavery system and it had plenty of elements of capitalism in it just by being part of a larger capitalist economy). Thus they were desperate to open up the west to slavery.

Up in the north, though, I don't think too many people really were focused on abolishing slavery in the south. The idea that the war was fought to abolish slavery is a myth (that's different from saying the war was about slavery, which it was). You had three slave states join the union cause, the emancipation proclamation wasn't declared until well into the war, and Lincoln himself I believe offered to let the slave states keep slavery forever if they stayed in the union (which again, considering what I said above, wasn't much of a consolation prize to the slavers who needed to be able to expand slavery into the territories). None of that is consistent with the union wanting to abolish slavery nationwide. Instead, what the northerners cared about was "Free Soil". The Free Soil Movement is critical to understand in order to understand the source of the conflict. We all know that America was founded on stealing indigenous land and using that as a "release valve" on class conflict. As we approach the civil war, the amount of land available for the taking was starting to get low. Everyone knew there would be a massive amount of genocide, ethnic cleansing, and then eventually new lands for further settler colonialism. But if slavery was allowed in these newly stolen lands, everyone knew that there was no way the yeoman farmer could compete with the plantation system. Not only that, but both the existing capital stocks as well as the super profits generated by the plantation system meant that the slavers could potentially buy up vastly disproportionate amounts of land, squeezing out the would-be yeoman farmers and capitalist farmers for that matter. The Free Soil Movement was the idea the idea that slavery should not be allowed in the territories, not because of moral indignation, but because both these would-be farmers as well as capitalist investors wanted it all for themselves.

So ultimately, I think a lot of the battle over the abolition of slavery came down to a power struggle between northern capitalism (with an understanding that they needed to keep the masses in north happy with the prospect of "free real estate) and southern slavery. Abolishment came as a way to break the political, social, and economic power of the slavers. Not dissimilar to how the emerging bourgeoisie in Europe had to overthrown the political, social, and economic primacy of the aristocrats. In Europe, the defeat of the aristocrats didn't mean they lost their wealth and power, just that they had to be subordinated. And likewise in the US, the plantation owners by and large still did fine for themselves after slavery was abolished; they just no longer were allowed to share power more or less equally with the industrial capitalists.