<Li> 1973--Gerald Ford became Vice President following Spiro Agnew resigning </Li> <Li> 1974--Nelson Rockefeller became Vice President following President Richard Nixon's resignation and Ford becoming President . </Li> <P> In addition, Section 3 of the Twenty - fifth Amendment provides that, whenever the President of the United States transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, the powers and duties of the presidency of the United States shall be discharged by the Vice President as Acting President . During such time, the Vice President would not be able to exercise his duties as President of the Senate, which would make the President pro tempore of the Senate the body's presiding officer . Section 3 has been invoked three times: </P> <Ul> <Li> July 13, 1985--President Ronald Reagan underwent surgery to remove cancerous polyps from his colon . Prior to undergoing surgery, he transmitted a letter to Speaker of the House of Representatives Tip O'Neill and President pro tempore of the Senate Strom Thurmond declaring his incapacity . Vice President George H.W. Bush was Acting President from 11: 28 a.m. until 7: 22 p.m., when Reagan transmitted a second letter to resume the powers and duties of the office . </Li> <Li> June 29, 2002--President George W. Bush declared himself temporarily unable to discharge the powers and duties of the office prior to undergoing a colonoscopy, which was done under sedation . He invoked Section 3 of the Twenty - fifth Amendment in letters given to White House Counsel Alberto Gonzales, who transmitted them by fax to Speaker of the House of Representatives Dennis Hastert and President pro tempore of the Senate Robert Byrd . Gonzales called Hastert's and Byrd's offices to confirm receipt of the letters, and then contacted Vice President Dick Cheney to advise him of the transfer . Cheney was Acting President from 7: 09 a.m. to 9: 24 a.m., whereupon Bush transmitted a second letter to resume the powers and duties of the office . </Li> <Li> July 21, 2007--Under the same circumstances as the 2002 invocation, President George W. Bush transmitted a letter to President pro tempore of the Senate Robert Byrd and Speaker of the House of Representatives Nancy Pelosi declaring himself temporarily unable to discharge the powers and duties of the office prior to undergoing a colonoscopy, which was done under sedation . Vice President Cheney was Acting President from 7: 16 a.m. to 9: 21 a.m. that day . </Li> </Ul>

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