I've seen a lot of northern-secession timelines, but a lot of them are close to ASB (severe Republican party fracture in early 1860 causing it to become six different parties) or don't involve much of the North seceding, as well as the secession being for a non-anti-slavery reason (I'm sure you all know which TL I'm referencing here). So, can anybody think of an actually plausible way that the North or a good portion of it (by this I mean more than three-quarters) might secede from the Union to combat slavery?