Maybe I'm being simplistic but I see the Constitution as the 'common denominator'. You can have a Union, and the 'landmasses' in that Union can do their own thing but there will come a time when more than one of thos 'landmasses' has a problem with each other or something/one else. The solution has to come from the Constitutional Law based on the Constitution they agreed to when becoming 'landmasses in a big mass'. Even a territory, once it belongs to the 'big mass' follow the Constitution of the 'big mass'.
If a 'landmass' drops out of the 'big mass', (and isn't obliterated by a MOAB) it makes a Constitution. Maybe it can co-exist next to the former partners. Maybe not. Maybe more than one has to band together to protect their common interest. If so, they write up a 'nother landmass' Constitution. Possibly based on the one they disregarded when they left the 'big mass'.
In all of that blather, it seems the Constitution is the common rule or toll taker to move through the 'land masses'. It's the interpretation of the Constitution that can get fuzzy. Like some politicians math.