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.
“moral objections”, Really not quite that cut and dried.
Up until Scott V. Sanford in 1857 The Abolitionists were pressing the Federal Government to take action to end slavery in the states. After the Scott decision abolitionists realized that their original demand for Government action to end slavery in the states would no longer work. So they shifted their demands to the Government limiting slavery to only those where it was legal and preventing slavery from expanding into the Territories of the United States. The Republican Party adopted this line in the 1860 election and Lincoln supported the concept. There was some moral objections to the expansion of slavery into the territories.
The reality is that it was mostly an issue of political power for the control of Congress.