Just to clear something up: Mike Pence did not technically become president when he took the oath at about 11:55 a.m. this morning. Nor would the nation have been without a president if Trump had taken his oath behind schedule.
The Twentieth Amendment to the Constitution, which moved Inauguration Day from March to January and established the current presidential line of succession, is clear on exactly when the new president and vice president take office:
The terms of the President and Vice President shall end at noon on the 20th day of January, and the terms of Senators and Representatives at noon on the 3d day of January, of the years in which such terms would have ended if this article had not been ratified; and the terms of their successors shall then begin.
The text in the middle there refers to the fact that Article II of the Constitution already
The presidential oath of office is in fact required and prescribed word-for-word elsewhere in the Constitution (Article II, Section one). But it isn’t like it’s a magical phrase that makes someone president. The president’s term begins at noon, whether he’s already taken the oath or he takes it a few moments later.
