To my knowledge, the US constitution doesn't allow a seccession, but it might not disallow it either. The EU is not a federation of states like the US or Germany, it's a contract among nations, just like e.g. the NATO, although with different incentives, of course. National law takes precedence over EU law, although contracts between nations require national laws to reflect it. With other words, either you change national law and regulations accordingly or you leave the union. I think that's fair. It's like either paying the bus ticket or leaving the bus. I doubt that US bus drivers are different than those here in Germany .