U.S. Constitution: 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.”

For those that are elected to the U.S. Congress or those that join the U.S. military or a federal law enforcement agency, there is a similar oath.

5 U.S. Code§ 3331.Oath of office

“An individual, except the President, elected or appointed to an office of honor or profit in the civil service or uniformed services, shall take the following oath: “I, AB, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true…