I’ve heard this claim a million times, yet despite the supposedly receding tide of slavery, the South was fighting tooth and nail to expand slavery into new territories.
True.
Because, if slavery was NOT expanded, the abolitionists would have control of Congress. When that state finally occurred, the representatives of these “Free” states imposed high tariffs and excises on overseas trade - which mostly impacted the South and favored Northern industries.
When faced with the reverse - a independent South with a 10% tariff, the North saw grass growing the deserted streets of New York and Boston - and no alternative but war.