I don't know how they knew that. 200 years later and I still believe that the Republican Party freed the slaves and began as the 'Abolition Party'...So I guess I could believe that 'Abolition' was just a cover for State's Rights, but that does not comport with the founding fathers and Christianty's anxiety over slavery.
Of course there was an economic issue, as there always is, but that does not negate the desire of white America to free the slaves.
It wasn't just slavery; slavery was part of the issue of course. It all came together.
Preserving the union was a bigger motivator in most minds than stopping slavery, though.