Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:--''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."A year ago, Stephen Gillon wrote at The Huffington Post:
There was a great deal of confusion on the plane and in Washington about a very basic constitutional issue: When did the vice president assume the powers of the presidency? Everyone knew the vice president succeeded the president in the event of death. But did LBJ become president when Kennedy was declared dead? Or did he need to take the oath before he assumed the powers of the presidency? No one was sure. (The opinion of the assistant attorney general was that Johnson assumed the title of president, but lacked the power of the office until after he took the oath. These issues would not be clarified until ratification of the Twenty-Fifth Amendment in 1967).
Johnson was no constitutional scholar, and the abstract debates about the oath were of little interest to him. At a time when the operating assumption was that the assassination was part of an international conspiracy, Johnson needed to make sure there was no ambiguity about who was in charge of the nation. Taking the oath in Dallas was the right thing to do.