The Constitution repeatedly refers to the Presidency as an “office” and requires the president to swear an “oath of office” before assuming the execution of his “office.” See, e.g., U.S. Const. art. I, § 3, art. II, §§ 1, 4; amends. XII, XXII, XXV. And it refers to an office “under the United States” in several contexts that plainly include the Presidency. See id. art. I, § 3, cl. 7 (Impeachment Clause), § 6, cl. 2 (Incompatibility Clause), § 9, cl. 8 (Emoluments Clause).