You would need a POD going back to the early days of the United States that allows slavery to spread to the Midwest, and flips the political atmosphere so that the agrarian slaveholders are more amenable to a strong central government. With these factors in play, one could imagine some sort of "reverse civil war" where the Free states (Pennsylvania on North, mabye including Michigan and Wisconsin), are increasingly radicalized by the increasing slaveholder power as plantations march further west, pass more intrusive fugitive slave laws, favor Southern infrastructure and free trade, and drag the North into unpopular wars against Mexico, and perhaps even Spain or Britain, to expand slavery. Eventually the Southern Democrats could elect a President that openly advocates making slavery legal throughout the entire nation, which scares the nascent northern industrial working class into fear that they'll be replaced with slaves (or just have to live alongside black people) and causes them to secede.
Even then it'd be a harder war for the Southern Union than OTL was for the North; there's still a ton of industry up there, the terrain is quite unfavorable, and Britain and France may be more willing to support a rebellion against slavery than for it. With enough luck however the South could win such a war and "conquer" the "North," which absent the Midwest never eclipsed the south to the degree of OTL.
It'd also be a rather dystopic US to say the least.