When Did It Become the FBI's Job to Act as a Check and Balance to the President? (RICH LOWRY, January 16, 2019, National Review)

It's literally their job: I, [name], 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 faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God.