Where it would not spread. What they were really worried about wasn't slavery spreading, it was the possibility that territories would turn into states favorable towards the Southern states and thereby affecting the balance of power in Washington DC.
That power balance is how they were getting Federal policy to flow money into their pockets, and this was always about making sure the money flowed into the pockets of the well connected "elite" of the North East.
Correct. There was no economic case for slavery spreading to the western territories. This was a power struggle between two sides. States meant Senators. The Senate was the real battleground. With its larger population the Northern states had long controlled the House. They needed to effectively rule over the Senate in order to force things like ruinously high tariffs through. So long as the Southern Senators could block that, the Southern states could at least somewhat protect themselves from Northern predation. The instant the Southern states left, they no longer needed seats in the Senate. They therefore did not need the territory of the United States and did not claim any of it. They hardly had some religious zeal to spread slavery. They were desperately fighting a rearguard action to try to prevent Northern special interests from really sucking the South dry.
DiogenesLamp: "Where it would not spread.
What they were really worried about wasn't slavery spreading, it was the possibility that territories would turn into states favorable towards the Southern states and thereby affecting the balance of power in Washington DC."
No, in fact, by 1860 slavery was still lawful in the Territories of Kansas, Oklahoma, New Mexico, Nebraska & Utah.
Even California, officially free, still had numbers of de facto slaves.
US States & Territories as of 1860:
Now DiogenesLamp wishes us to believe that Northerners only opposed slavery for economic self-interest reasons and Republicans only opposed slavery for political self-interest reasons.
But the truth is that Northerners first became anti-slavery in their churches, for moral reasons, and culturally through books like "Uncle Tom's Cabin".
Neither of those had anything to do with economic or political self-interest.