In the 1860s, the central national political issue was the debate over slavery. In 1860, a new party won the presidency of the old United States, electing Illinois politician Abraham Lincoln on a platform of stopping the spread of slavery to new states and territories. He was elected largely on Northern moral outrage over the outcome of the Supreme Court case Dredd Scott v. Sanford, which made laws prohibiting slavery in federal territories unconstitutional.
A lesser known fact, though, is that many Southern states flirted with secession even though they controlled the US Senate, and had a pro-Southern majority on the Supreme Court. Nevertheless, cooler heads prevailed, and Lincoln's term was fraught with difficulty in accomplishing anything, let alone his elected mandate.
When the US Supreme Court ruled in Lemmon v. New York, that states had no power to abolish slavery within their borders or emancipate slaves in transport, that was the straw that broke the camel's back, and began the process that led to all of the free states seceding from the United States and forming a new federal republic.
How do we reverse this outcome, and make it so that the Southern states are the ones who feel they have the most to gain from secession? What kind of government would they form, and how would the rest of the US react to this? Would it mean civil war, unlike IOTL?