It was about political power in Congress. The South had been losing it’s influence as geographic region since the 1840s.
It was exactly about this. I had been taught all my life that it was about moral objections to slavery in the territories. I only learned in the last few years that "moral objections" were just astro-turf, and the real bone of contention was control of congress.
The South had been losing power, but they were still producing most of the taxes, and most of the shipping business into and out of New York.
They were only getting about 40% of the total revenue their exports produced, and the rest was going to New York and Washington DC. With no ability to control congress, they could change no laws that would impact the existing circumstance.
Yes. "Expansion of slavery" really mean "expanding Southern power in congress, and nobody was really talking about moving large numbers of slaves into the territories.