Race had never been a defining element in successful nation states. The true definition always depended far more on distinctions in language, culture… - 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.

The South had a far better chance of preserving its institutions and quirks of culture by remaining a part of a larger nation.

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 government.