When the war started, it was “Slavery forever!”, and after they were whipped, suddenly it was “States’ rights!”

Easiest political argument to slam dunk. Point to the Articles of Secession that the states sent to Congress explaining why they were leaving. I forget the numbers, but at least 2 point to slavery as the #1 reason, and I believe they all mention it as a top issue.

https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states

To really torque you opponent off, start with Mississippi’s letter:

“Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world.”