Thank you for the link. I had not read this until today. I think Paulson makes a great case, and yes, the president is only obligated to enforce constitutional laws. Congress also has an obligation to only pass constitutional legislation and the during the ratification debates several proponents of the Constitution argued that should the general government violate the Constitution, the States would be “powerful enough to check it,” in other words nullify unconstitutional legislation. James Wilson essentially made such a statement in his famous State House Yard Speech of Oct 1787.