The slave states would have have said that this was just bolstering an existing, common understanding that the US was voluntary association of states, and hence

The slave states would have have said that this was just bolstering an existing, common understanding that the US was voluntary association of states, and hence even the US constitution could not override basic state powers. And before the Civil War, that understanding might well have held in both the North and the South. Indeed northern states often thumbed their noses at the federal fugitive slave laws -- including the Constitution. By the time of the war though, the political state of play was that state sovereignty and the abolition of slavery were at loggerheads -- and abolishing slavery was far more important.