What did the slavery anti-abolitionists fear emancipation would produce in the U.S.? What did they specifically warn against?
Pro-slavery forces thought that the Black man were too uncivilized to live in society. The implication was they they would be too lazy to work unless someone held a whip over them, and that they would commit crimes against white people.
Some also worried about the economic loss; slaves were property, and the loss of slaves meant their owners lost a lost of money.
It went a lot farther than that. Not only would there be the immediate loss just from the value of the slaves, but the prices of things like tobacco and cotton would rise, which would make those crops much less attractive to their European consumers. This would be a huge loss for the plantations, and since the entire southern economy was heavily based on agriculture, the entire economy of the south would be certain to suffer greatly.
You also have to remember that the Civil War wasn’t just about slavery. A lot of it was about control. For example, a lot of the smaller farmers in the south would have been happy to get rid of slaves, because they couldn’t afford them and didn’t have any. The small farms had a hard time competing against the big plantations because the big plantations had cheap slave labor. However, these same small farmers also were sick and tired of the northern industrialists trying to destroy the southern agriculturalists. They feared that getting rid of the slaves would give the northern industrialists control of the country, and agriculture in general would suffer. Many of these small farmers happily jumped onto the anti-abolitionist bandwagon not so much because they were anti-abolitionist but because they were anti-industrialist.
(I hate it when I think of more to say after I already hit enter - sorry for the double reply)
Also, a lot of folks at the time (even many of the abolitionists) thought that blacks were inferior to whites, especially intellectually. A lot of white supremacists feared that allowing blacks to be free men would also allow them to intermix in society, and if blacks were allowed to breed with whites, then the white race would be diluted and would lose some of its intellectual superiority.
Another pro-slavery argument (though not really a fear) was that black slaves had a better life than free factory workers in the north. Considering that this was the age before a lot of labor laws, and some southerners actually did take fairly good care of their slaves, this may have actually been true in some cases. In many cases though, slavery was just brutal, and I’m sure those slaves certainly would not have agreed with that argument.
They also feared that a free low class would result in class warfare, which would destabilize things both politically and socially. Keeping the lowest part of society enslaved helped keep society stable and prevented lower class uprisings and unrest.
A lot of people feared a race war.