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Like I said, slavery was on the decline in states where slaves were needed less because of changes in crops.

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.

Smokin' Joe: "It wasn't a question of abolition, but economics.
Just as the war was a question of economics."

But in 1860 there was no doubt -- none -- in the minds of Deep South slave-holders about the economic values of slavery.
They said so, often and proudly.

From Mississippi's Declaration of Causes for Secession, January 1861:

"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world.
Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth.
These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun.
These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization.
That blow has been long aimed at the institution, and was at the point of reaching its consummation.

There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin. "

In 1860 next to land itself, slaves were the US's single biggest asset class, valued at over $4 billion, far more than all the Northern industry or railroads combined.
By 1860, Deep South economic growth had been continuous and unparalleled for 20 years, enhancing slave-based prosperity which went back to the earliest settler days.
Southerners were fully aware of their economic and political powers, and were determined to do nothing to jeopardize them, until...

Until the election of 1860 brought "Black Republicans" like Abe Lincoln to office in Washington, DC...