What were the concerns about allowing or banning slavery in newly established territories during the 1800s?

The issue was states rights. slavery is not discussed in the constitution until the 13th amendement. since it wasn't in the constitution it was a decision that belonged to the states. if the balance of slave and free states isn't kept then the government was showing if favored one side over the other when they didn't officially have a position on the subject.

A. southerners underestimated republicans eagerness to end slavery.