See my post #226 above.
In summary: Northern states had gradually abolished slavery beginning with Vermont in 1777, such that by 1860, very few remained.
But the Northern revolt against Federally enforced slave laws began with the 1850 Fugitive Slave Law, shifting responsibility for returning slaves from Northern states to the Federal Government.
The Northern revolt then picked up steam after the 1857 Dred Scott decision, effectively making slavery lawful in every state, North, South, slave or allegedly "free".
That's what destroyed the old Whigs and ignited the new Republican Revolution, which elected its first president in 1860, Abraham Lincoln.
Smokin' Joe: "Slavery would have ended in some states of its own weight. Crop changes such as those happening in Maryland from tobacco..."
Maybe, but probably not.
In fact, there seems to have been a firewall against abolitionism along the Mason-Dixon line.
To cite one example: Delaware.
Delaware had fewer slaves percentagewise than any other slave-state, had large numbers of free-blacks, refused to join the Confederacy, supplied far more troops to the Union than Confederacy, and yet, and yet...
Delaware refused to abolish slavery until forced to do so by the 13th Amendment.
Maryland was similar to Delaware in having many free-blacks and anti-slavery citizens.
But there was no pre-Civil War movement in Maryland to abolish slavery itself.
Finally, all the talk you hear about modern machines making slavery unprofitable us just nonsense.
What slaves by 1860 had fully demonstrated was that they could operate, or help build any machine their white masters could devise.
So there was nothing about machinery, per se which necessitated abolitionism.
Bottom line: slavery was a cultural thing, which only changing cultural values could abolish.
How likely was that in the Deep South? Never.
How likely was that in the Upper South? Maybe, someday, distant future.
How likely was that in Border States like Delaware & Maryland? Could be, by end of 19th century.
If someone had devised a machine to pick cotton, there would have been a wholesale manumission overnight. Because it was expensive to keep slaves. For the same reason the automobile overtook the horse: you don't have to feed the machine when it isn't working. There was another trend that made it convenient for the North to remove its negroes, that of immigration from Europe. Why own a slave when you can work the Irish cheap, keep them living in poverty and desperate for work so they will work for nothing, and do the most dangerous jobs out of desperation--jobs no slave would be given, because they might be killed and deprive the owner of their investment.
It wasn't a question of abolition, but economics. Just as the war was a question of economics.