Nothing. People really want this to be a thing, and those folks are just wrong. The Constitution says two things and two things only: The date and time Presidential terms start and end, and the oath that must be taken sometime before the new President assumes duties of the office.
Article II, Section 1, Clause 8:
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.
Washington took office on March 4 but not take the oath until April 30 - This was before the 20th amendment moved the date and time to noon on January 20th. Kennedy wasn't sworn in until 12:51. Did we not have a president for 51 minutes? No, though this hasn't been legally tested as far as I know.
Swearing/affirming In doesn't make them President - the date and time do. To DO anything as President though, constitutionally, they must swear or affirm the Oath.
Nothing. People really want this to be a thing, and those folks are just wrong. The Constitution says two things and two things only: The date and time Presidential terms start and end, and the oath that must be taken sometime before the new President assumes duties of the office. Article II, Section 1, Clause 8:
Washington took office on March 4 but not take the oath until April 30 - This was before the 20th amendment moved the date and time to noon on January 20th. Kennedy wasn't sworn in until 12:51. Did we not have a president for 51 minutes? No, though this hasn't been legally tested as far as I know.
Swearing/affirming In doesn't make them President - the date and time do. To DO anything as President though, constitutionally, they must swear or affirm the Oath.