It meant to the Northern section indefinite Southern ascendency, prolonged Southern lead in national legislation.

All the smouldering passions of the earlier period, of embargo, and non-intercourse, and the war of 1812, flamed suddenly and fiercely in the heart of the free States.
The length and bitterness of that controversy excited the gravest apprehensions for the stability of the Union.

The dread of disunion led to mutual concessions, to the Missouri Compromise.

The slave-holding section got its immediate claim allowed, and the free States secured the erection of a line to the north of which slavery was forever prohibited.
And besides this, the admission of Maine was supposed to neutralize whatever political advantages, which would accrue to the South from the admission of Missouri as a slave State.

Both sections were content, and the slavery question was thought to be permanently settled.