Own slaves.
The South wanted to expand slavery to the territories, the North didn't.
If Southern Democrats believed so much in 'states rights' they wouldn't have pushed the 1850 Fugitive Slave Act through Congress.
It was not practical to "expand" slavery into the territories. I was taught that crap in school, and a few years ago I decided to look at it, and what I discovered is that you can't grow cotton in any territories without modern irrigation systems that didn't exist in the 1860s, and therefore there would never have been any significant slavery in the territories because there was no money in it.
A slave in 1860 was valued at about $100,000.00 in today's money, and there was a substantial return on investment in cotton plantations and some in tobacco and indigo, but after that, the profits fell off quickly.
There never would have been plantations in the territories, and so there never would have been any significant slave presence in the territories.
If you look up the Wikipedia entry on "New Mexico Territory" you will notice that when "New Mexico Territory" encompassed all the land between Texas and California, they had less than a dozen slaves in all those millions of acres of territory.
If Southern Democrats believed so much in 'states rights' they wouldn't have pushed the 1850 Fugitive Slave Act through Congress.
People get stuck on that "fugitive slave act" while seeming completely unaware that Article IV, section 2 of the US constitution says the same thing as the "fugitive slave act."
Are you suggesting states defy constitutional law? How is that different from "rebellion"?