The Legislature should not pass laws without sincerely believing they are Constitutional, and the Executive should not sign laws without believing they are Constitutional. Bush 43 violated this -- he thought McCain-Feingold was unconstitutional but punted the hard unpopular decision to Someone Else.
I was talking more about laws that have already been written and signed since we were talking about the actions of the FEC in the context of McCain-Feingold. Absent extreme cases (like death camps or something), I don't think the FEC or its employees would have any legal right (or responsibility) to decide for themselves that McCain-Feingold was unconstitutional. Giving them that power and responsibility is the can of worms I was talking about.