In 1860, Karl Marx had a preference for the Republicans. He exchanged letters with Abraham Lincoln.
But, also to show that Marx/Engels' ideas were scarcely original: long before the Civil War, Southerners were calling the free states hypocrites, as even though they didn't have slavery, they allowed employers to exploit the labor of 'wage slaves.'
Their claim that their paternalistic slavery in which the slaves were fed and lodged was superior to the the wage slavery in which workers were paid money but then left to fend for themselves was the South's ideological justification for the Civil War.