Slavery was legal, although immoral, it's legacy was part of our country racist past North and South. The manufacturing culture of the North didn't need the slave model so in the early 19th century they sold all their slaves "down the river" to the south. Then they (Northern mercantile class) raised tariffs on imports after making money on selling their slaves to the south. Then they turned against that "peculiar" institution on moral grounds making the hypocrisy factor unbelievable.
FYI - thousands of blacks in this country were slave owners. Some were slave breeders, selling their own offspring into slavery.
Slavery was about money and selfishness, not racism.
Not only was that mostly true (tariffs were national decisions which the South generally supported). But many of the fortunes of the earliest Northern industrialists came from the slave trade either directly or indirectly.
There were areas of NY state where slavery was as concentrated as in the South in the late 1700s. But that was an anomaly and most of the slaves in the North were dispersed in small numbers among the general population not held in large groups.
But for the invention of that Connecticut Yankee, Eli Whitney, slavery may well have been phased out in the South as is was in the North. It was an archaic institution unsuited to a modern economy.
However, much of the North was opposed to Lincoln taking the slavers on even after federal institutions were attacked. That part of the industrial class that depended upon cotton was vehemently opposed to the War. The cities controlled by the Democrats were opposed.
BINGO!