He actually took an oath to protect and defend the Constitution. "I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States." That being said I do agree that the president has no authority to legislate or change law. He shouldn't even be suggesting things to congress. His job is to execute the laws that congress passes and act as commander in chief after war has been declared.