An acting president of the United States is an individual who legitimately exercises the powers and duties of the president of the United States even though that person does not hold the office in their own right. There is an established presidential line of succession in which officials of the United States federal government may be called upon to take on presidential responsibilities if the incumbent president becomes incapacitated, dies, resigns, is removed from office (by impeachment by the House of Representatives and subsequent conviction by the Senate) during their four-year term of office; or if a president-elect has not been chosen before Inauguration Day or has failed to qualify by that date.
Presidential succession is referred to multiple times in the U.S. Constitution – Article II, Section 1, Clause 6, as well as the Twentieth Amendment and Twenty-fifth Amendment. The vice president is the only officeholder named in the Constitution as a presidential successor. The Article II succession clause authorizes Congress to designate which federal officeholders would accede to the presidency if the vice president were unable to do so, a situation which has never occurred. The current Presidential Succession Act was adopted in 1947 and last revised in 2006. The order of succession is as follows: the vice president, the speaker of the House of Representatives, the president pro tempore of the Senate, and then the eligible heads of the federal executive departments who form the president's Cabinet in the order of creation of the department, beginning with the secretary of state.
The vice president immediately assumes the presidency in the event of the death, resignation, or removal of the president from office. Likewise, were a president-elect to die during the transition period, or decline to serve, the vice president-elect would become president on Inauguration Day. A vice president can also become the acting president if the president becomes incapacitated; should the presidency and vice presidency both become vacant, the statutory successor called upon would not become president but would only be acting as president. To date, three vice presidents—George H. W. Bush (once), Dick Cheney (twice), and Kamala Harris (once)—have served as acting president. No one lower in the presidential line of succession has so acted.