There were more radical abolitionists calling for secession for years before the Civil War, and the South was politically dominant for decades. With that in mind, it would be entirely within the realm of possibility for things to skew slightly more in the slave states' favor and provoke the North into seceding. The outcome of such a war is more up in the air than OTL, but I feel like the same factors that allowed the North to win IOTL would still work in their favor, especially since the fact they explicitly oppose slavery raises their chances of gaining international recognition.