I'm guessing this is going to lead to a Deadlands and Wild West Exodus situation.
Pretty much what I was going to say. The idea is very Deadlands. However, the Deadlands south was propped up by magic ghost rock mad science.
The way things go depend on if magic requires training or is inherent and something you could learn without a guide. In D&D terms: Wizard vs. Sorcerer respectively.
If magic people just automatically have magic and even 1% of enslaved people have it: the South is fucked from the inside.
If it requires training, well most educational institutions are in the Northeast and the greater population: The South is fucked as soon as they rebel.
If we're going with World of Darkness and Werewolves are northerners and Vampires are in the south, no reason to hide means no reason to go full crinos and terrify the mortals into fleeing by their presence alone as they fillet leeches: the South is fucked.
If werewolves and vamps are evenly split (not that werewolves would willingly side with leeches), the north still has technological and numerical advantages on their side: The South is fucked.
Changlings (dreaming or lost) would most likely not get involved because of their own court disputes, leaving the whole thing to everyone else: The South is fucked.
Bring in changing breeds and...
You get what I'm saying here. That's not even getting into the more obscure varieties of WoD books. Prometheans would trash both armies.
Unless a whole bunch of dragons work for the South, the North wins. Might be a bit bloody, might be a bit longer, but attrition would bleed the South dry. There is no logical scenario, without all magicals inexplicably siding with the South, that has the South winning.
What would be a more interesting story would be World of Darkness meets West Wing: a highly political story of vampire politicians, changling lawyers, werewolf generals all maneuvering through 1850s-60s DC shoulder to shoulder with mortal politicians.