So long as the number of slave states was the same as or greater than the number of free states, then in the Senate the South had a check on the gove… - William C. Davis

Inextricably intertwined in the question was slavery, and it only became the more so in the years that followed. Socially and culturally the North and South were not much different. They prayed to the same deity, spoke the same language, shared the same ancestry, sang the same songs. National triumphs and catastrophes were shared by both. For all the myths they would create to the contrary, the only significant and defining difference between them was slavery, where it existed and where it did not, for by 1804 it had virtually ceased to exist north of Maryland. Slavery demarked not just their labor and economic situations, but power itself in the new republic.

Southern states had embraced the Union only insofar as it served to protect their rights to hold property in slaves, and to spread slavery as the nation expanded and the institution itself became intertwined as a defining element in the struggle for national power itself. In slavery could not spread as new states were formed, then the existing slave states would be doomed to perpetual minority in representation in Congress, guaranteeing that if the day came when Northern antipathy to slavery itself became hot enough, the majority could use the government to subvert the Constitution and abolish the institution where it already existed. In short, the South could not affort to lose any battle over slavery, nor even over issues on its periphery.

In fact, the state rights defense of secession in 1860–1861 did not really appear in force until after 1865 as builders of the Lost Cause myth sought to distance themselves from slavery.