The 25th Amendment to the Constitution addresses the subject of presidential succession in the event of the death or disability of the President. It makes it clear that the vice president becomes president if the president dies, resigns, or is removed from office, and it establishes how a vacancy in the office of the vice president can be filled. It also provides for the temporary transfer of the president's powers and duties to the vice president, either on the initiative of the president alone or on the initiative of the vice president together with a majority of the president's cabinet. In either case, the vice president becomes acting president until the presidential powers and duties are returned to the president.
The 25th amendment was submitted to the states on July 6, 1965, by the 89th Congress and was adopted on February 10, 1967, the day that the requisite number of states (38) had ratified it.
Section 3 of the amendment allows for the voluntary transfer of presidential authority to the vice president (for example, if the president is undergoing a medical procedure). This is accomplished by the president declaring in writing that he is or will be unable to discharge the powers and duties of the presidency. The vice president then assumes those powers and duties as acting president. The vice president does not become president and the president remains in office, but during the time that the voluntary transfer is in force, the president relinquishes his presidential authority. The president regains those powers and duties upon declaring, in writing, to be again able to discharge them.
In Reagan's case, the surgery lasted just under three hours and was successful. Reagan resumed the powers of the presidency later that day. The previous day, July 12, 1985, Ronald Reagan underwent a colonoscopy and was diagnosed with bowel cancer. He elected to have the lesion removed immediately. He consulted with White House counsel Fred Fielding about whether to invoke Section 3, and in particular about what kind of precedent would be set if he did so. Fielding and White House Chief of Staff Donald Regan recommended that Reagan transfer power. Two letters were drafted: one specifically invoking Section 3, the other mentioning only that Reagan was mindful of its provisions. Reagan did not formally invoke section 3. On July 13, Reagan signed the second letter before being placed under general anesthesia for a colectomy. Vice President George H. W. Bush was acting president from 11:28 a.m. until 7:22 p.m., when Reagan transmitted a letter declaring himself able to resume his duties.
Reagan spent a week convalescing at the hospital before returning to the White House. After the surgery, Reagan quipped, “Well, I’m glad that’s all out.” He told reporters that he planned to live a long time. At the time Reagan was 74, and was then the nation’s oldest serving president.
Although Reagan never formally invoked section 3, Fordham Law Review commentator John Feerick argues that this is in fact what Reagan did. He posits that although Reagan disclaimed any use of the Twenty-fifth Amendment in his letter, he followed the process set forth in Section 3. Feerick argues that "no constitutional provision except the Twenty-Fifth Amendment would have allowed" him to designate the vice president as acting president.
In his memoir, Reagan later agreed with this position and stated that he had in fact, invoked the Twenty-fifth Amendment.