What the LEO in question does not realize is that he is caught in a circular argument.
Any LEO who sets aside the Constitution by enforcing unconstitutional laws, is in essence saying that the Constitution no longer has authority, the very same authority by which his office derives the just consent of the people, thereby nullifying his authority and making his actions illegal.
So how does he resist enforcing a law he believes to be unconstitutional, but has been endorsed by SCOTUS?