Oath of Office
Article II, Section 1 of the Constitution provides an oath of office for the president. The president must solemnly swear (or affirm if he or she cannot swear for religious reasons) to faithfully execute the Office of President of the United States and will, to the best of his or her ability, preserve, protect and defend the Constitution of the United States. All other federal officers take a different oath, created by statute rather than by the Constitution, where they swear (or affirm) that they will uphold the Constitution of the United States.
By Donald Ritchie, Our Constitution (Oxford University Press)