What it says. Your challenge is to create a situation in which an abolitionist North secedes from a vehemently pro-slavery Union or something similar.
and...GO!
and...GO!
I know a timeline based on this idea by Robert Perkins called what hath a warm overcoat wrought?
I actually thought about doing a scenario like this in my timeline. I don't think it's going to work out that way now, but this is how I would have done it:
The United States fights a war with Britain over the Oregon Country in the late 1840's. The United States loses, and the North suffers most of the damages: most of the land war fought around the Great Lakes and New England, naval blockade & bombardment, and so on. The South gets off easy (no invasion, less stringently-enforced blockade). This leads to the North being greatly embittered at the South, and also helps bring about the emergence of a much more coherent Northern regional identity as a counterpart to the already-strong Southern one. Both sides of the Mason-Dixon line are now more inclined to think of themselves as different peoples with different interests. Eventually, some kind of dispute over slavery happens; maybe the abolitionists get the idea of appropriating the idea of nullification to combat a Fugitive Slave Act? Their refusal to follow the federal laws regarding fugitive slaves leads them to secede from the slaveholding United States and found their own country.
Probably all the free states secede in such a scenario, and all the slave states remain in the United States. Depending on how the Mexican Cession has happened ITTL, we might see most of the western territories stay in Southern hands (remember, the North is much weaker ITTL and may not be able to hold them). So we could end up with a DOD-like United States that still reaches to the Pacific and a Northern Republic centred around the Atlantic coast and Great Lakes that extends only to the Rocky Mountains (say, from the OTL Kansas-Nebraska border north). If the Mexican Cession never happened in any form, we might even have a TL where neither American republic extends to the Pacific, too.