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.
Except that during the 1850s slavery was lawful & practiced in the western territories of Kansas, Oklahoma, New Mexico, Utah & Nebraska.
In Kansas especially there was competition between slaveholder settlers and free-soilers in which for years slaveholders held the political whip-hand.
Also in New Mexico, Oklahoma & Utah slaveholders held the political upper-hand.
Even California, nominally a free-state, held large numbers of de facto slaves until the Civil War, and appointed pro-slavery US Senators, notably Gwin & Weller.
So there's no doubt that for Democrat slaveholders, political power was all-important because without it slavery was doomed.
And that such Democrats would project their own motives onto Republicans is, at least, understandable -- they still do it today.
But the fact remains that most grass-roots Republican voters were first motivated by books such as the Bible and "Uncle Tom's Cabin" -- so their feelings were first Christian moral & cultural, not just Marxist economics & Democrat politics.