Exactly what it says on the tin. Make the Union have it's split earlier than it did in OTL
There are several rules in this challenge. First, the Civil War has to take place in the early 1850s at the latest. I'll set the mark at before Franklin Pierce became President. In the late 1850s, simply having Frémont win in the '56 election would do the trick.
Second, no North-secession sceneros à la Decades of Darkness and no having the Union break up in it's infancy. I want the civil war to be under similar circumstances to OTL (albeit with differences due to the earlier time), i.e. it should be about slavery.
Have fun!