I Guess He Just Forgot...
Hmmm. . . What's wrong with this picture?
Presidential Oath of Office:
"I, name, do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and I will to the best of my ability, preserve, protect, and defend the Constitution of the United States."
The Constitution of the United States of America (referred to above, in the Presidential Oath of Office), Article IV, Section 4:
"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion. . ."