I've heard this "slavery spread" theory all my life and I used to believe it until I wondered what a modern cotton map looked like.
What I discovered is that slavery could not have spread. Neither Cotton nor Tobacco will grow in any of the territories slavery could have expanded to, at least not for another 60 years or so it would take to make modern irrigation pumps.
The simple truth is that in 1860, slavery could not spread. It was literally impossible.
cotton grows in California I think they still have fields in so cal, in Kern county.
California. California’s cotton is mostly grown in seven counties within the San Joaquin Valley, though Imperial Valley and Palo Verde Valley also have acres planted. In the 1990s cotton was also planted in the Sacramento Valley. California is the largest producer of Pima cotton in the United States.
Why do you associate expansion of slavery with growing cotton. Surely slaves can be put to other uses.