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 solemly 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.
What is the time relationship between a President’s assumption of office and his taking the oath? Apparently, the former comes first, this answer appearing to be the assumption of the language of the clause. The Second Congress assumed that President Washington took office on March 4, 1789, 1 although he did not take the oath until the following April 30.
That the oath the President is required to take might be considered to add anything to the powers of the President, because of his obligation to
preserve, protect and defend the Constitution, might appear to be rather a fanciful idea. But in President Jackson’s message announcing his veto of the act renewing the Bank of the United States there is language which suggests that the President has the right to refuse to enforce both statutes and judicial decisions based on his own independent decision that they were unwarranted by the Constitution. 2 The idea next turned up in a message by President Lincoln justifying his suspension of the writ of habeas corpus without obtaining congressional authorization. 3 And counsel to President Johnson during his impeachment trial adverted to the theory, but only in passing. 4 Beyond these isolated instances, it does not appear to be seriously contended that the oath adds anything to the President’s powers.