two years (24 months) of another president's four-year term. Harry S. Truman, the president at the time it was submitted to the states by the Congress, was exempted from its limitations, and briefly sought a second full term—to which he would have otherwise been ineligible for re-election, as he had been president for more than two years of Roosevelt's fourth term—before he withdrew from the 1952 election. Since becoming operative in 1951, the amendment has been applicable to six twice-elected presidents: Dwight D. Eisenhower, Richard Nixon, Ronald Reagan, Bill Clinton, George W. Bush, and Barack Obama. Vacancies and succession Under Section1 of the Twenty-fifth Amendment, ratified in 1967, the vice president becomes president upon the removal from office, death, or resignation of the president. Deaths have occurred a number of times, resignation has occurred only once, and removal from office has never occurred. The original Constitution, in Article II, Section 1, Clause 6, stated only that the vice president assumes the "powers and duties" of the presidency in the event of a president's removal, death, resignation, or inability. Under this clause, there was ambiguity about whether the vice president would actually become president in the event of a vacancy, or simply act as president, potentially resulting in a special election. Upon the death of William Henry Harrison in 1841, Vice President John Tyler declared that he had succeeded to the office itself, refusing to accept any papers addressed to the "Acting President," and Congress ultimately accepted it. This established a precedent for future successions, although it was not formally clarified until the Twenty-fifth Amendment was ratified. In the event of a double vacancy, Article II, Section 1, Clause 6 also authorizes Congress to declare who shall become acting president in the "Case of Removal, Death, Resignation or Inability, both of the president and vice president". The Presidential Succession Act of 1947 (codified as ) provides that if both the president and vice president have left office or are both otherwise unavailable to serve during their terms of office, the presidential line of succession follows the order of: speaker of the House, then, if necessary, the president pro tempore of the Senate, and then if necessary, the eligible heads of federal executive departments who form the president's cabinet. The cabinet currently has 15 members, of which the secretary of state is first in line; the other Cabinet secretaries follow in the