When you really look at it. Neither of the larger elements of either side Unionist or Confederate saw the issue of slavery as a "National Issue". Slaves were considered as Property and thereby governed by the 4th Amendmend as such. The Southerners saw the slavery question as a Constitutional Issue regarding the authority of a National Government to direct. or mandate, the individual states to impliment anything. Much like a parrallel with our present National Health Care debate and the government demanding that we buy insurance, or face a penality fine, or possible imprisonment. The Southern states simply said that the National Government did not have that authority. And if we are still arguing that point today, I would say that the southern responce was valid.
So the Unionist issue was to Preserve the Unity and solidarity of the nation. Or else they would have been violating the very Constitution and their own states sovergnity. Something that the New Englanders were very protective of.