You swear on it when you take the Oath Of Office.
"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."
However,
The Constitution provides no standards for determining whether a President has violated their oath. The fact that other branches interpret the Constitution, and may do do inconsistently with the President, creates difficulties in determining whether the oath has been violated. Just as some Presidents have suggested that the oath may require them to disregard laws when doing so is necessary to preserve, protect and defend the Constitution, some lawmakers have argued that the President’s oath requires them to execute all laws, regardless of whether the President believes them to be constitutional.
The Supreme Court has not addressed these competing views, and the oath and its surrounding text do not suggest that questions about violations of the oath were intended for judicial resolution. The Court has held that the President is generally immune from civil or criminal liability for official actions taken while in office, which may impede judicial resolution of questions relating to a President’s violation of their oath arising during the President’s tenure. The Constitution’s justiciability requirements are another potential obstacle to resolution in federal court.
Impeachment provides a vehicle by which Congress may adjudicate a President’s alleged violation of their oath. Articles of impeachment against Andrew Johnson charged the President with being unmindful of the high duties of his office and of his oath of office. Draft articles of impeachment to be used against President Richard Nixon alleged that President Nixon violated his oath, though he resigned before these articles were adopted. Articles of impeachment adopted in the impeachment of President Bill Clinton charged the President with violating his constitutional oath, as did articles of impeachment adopted in both impeachments of President Donald Trump.
The political process provides another check on the President’s violation of their oath. James Madison and Alexander Hamilton suggested in various contexts that political accountability might help ensure the President’s fidelity to their office. In his second inaugural speech, George Washington observed that violating his oath would invite the upbraidings of all who are now witnesses of the present solemn ceremony.
https://constitution.congress.gov/browse/essay/artII-S1-C8-1-5/ALDE_00013936/
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