The north wasn't as economically dependent on slavery as the South but was just as supportive of it. In fact Massachusetts was the last state to do away with slavery.
If slavery has been the real issue all the northern states would have been slave free well before the end of the War of Northern Aggression BUT THEY WEREN'T.
It was and emotional bit of propaganda to hide Lincoln Monarchical aims.
That would have been New York in 1827. From a strictly legal standpoint it actually would have been Kentucky and Deleware and those parts of the southern states controlled by the North when the Emancipation Proclamation became effective. Slavery was theoretically legal there until December 1865.
Talk about a dumb lie!