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.