Also tell them NYC was one of the, if not number one, major port for unloading slaves onto US land. Another thing to make their heads explode - the ending of the slavery was through amendment. If it was the 3/4 state vote as the final decider then it means the 'government overreach' fall flat.
Another thing about proto-democrats and certain types of people in the NE states like Massachusetts is that quite a few of them weren't the freedom lovers that Washington, Franklin, and the rest of them were. Those other people were in it only to protect their own wealth and self interests. Out loud they talked the talked but inwardly they didn't care which side won as long as their wealth and property was intact. Those type of people should've been shipped back to England or wherever along with the Redcoat army.