One of the hardest concepts to teach 8th graders about the Civil War, is the fight after Slave vs Free States and how it affects their power in Congress. This breaks it down slowly!
I discuss that each state wants to make laws. There are two ways to do this #1 Have Representatives (based on population) or #2 Have Senators (Hopefully you covered how a bill becomes a law earlier in the year, if not maybe review)
I discuss why the South doesn't have as many Representatives because of lower white population (see "US Population in 1850 assignment") in an earlier lesson. (You could also address the idea of the 3/5 Compromise, too!)
The South relied on getting more states... therefore getting more senators in Congress. The fight was over which new states were Slave or Free States.
It also covers the Missouri Compromise, Compromise of 1850 and Kansas-Nebraska Act