> However, the case in the States was that at the turn of the 19th Century, the States were all pretty much fine with slavery.
No, they weren't. The protections of slavery written into the original constitution (including the fact that slaves, despite not being property rather than represented citizens, were counted at all in apportioning House seats -- and therefore also electoral votes) were written into it specifically because at that time the slave states of the South were already convinced that the North -- and the majority of the represented citizenry of the country -- would be inclined to at least greatly restrict if not abolish it in short order, based on the abolitionist movements that actually existed at that time and had been successful in the North, some even before the Revolution.
The North tolerated slavery in the South as the price of Union, right up until the South decided to sunder the Union over it, but the idea that the whole country was "pretty much fine with slavery" even at the turn of the 19th Century is just plain historically indefensible.