In the history of the United States, 14 vice presidents have gone on to become president. Of these, only John Adams, Thomas Jefferson, Martin Van Buren, Richard M. Nixon, and George Bush gained the office through election after their term(s) as vice president. Eight men became president following the deaths of their predecessors from either natural causes or assassination. They were: John Tyler, Millard Fillmore, Andrew Johnson, Chester A. Arthur, Theodore Roosevelt, Calvin Coolidge, Harry S. Truman, and Lyndon B. Johnson. Gerald Ford became president under complicated circumstances involving first the resignation of Vice President Spiro T. Agnew, then the resignation of President Richard Nixon. The vice presidents who became president due to either death or resignation are often known as the "Accidental Presidents." They were men suddenly thrust into the presidency and upon the American public.

For every accidental president, there were new questions regarding constitutional technicalities and the proper behavior of a vice president who had inherited the Oval Office. For example, should an accidental president be known as the vice president, the president, or the acting president? Should an accidental president work to further the agenda of the previous president or support his own initiatives? Though the questions of presidential succession have been debated since the founding of the republic, for the moment they have been put to rest by the 25th Amendment. Each of the eight accidental presidents faced his own complications and controversies.

As the first vice president to assume the office of president upon the death or removal of a sitting president, John Tyler was responsible for setting many precedents for the procedures—and problems—of the presidential succession. When William Henry Harrison, the ninth president of the United States, died one month into his term from pneumonia contracted during a record-length inaugural speech given in a freezing rain, no one was quite sure what to do. The Constitution did provide for the responsibilities of the president to be filled by the vice president, so Tyler was next in line. However, the Constitution made no clear distinction as to what Tyler should be called. Tyler insisted upon assuming the official title of president of the United States, rather than that of acting president. The Cabinet and Congress agreed. Both the House of Representatives and the Senate passed resolutions recognizing him as the president, and he took the presidential oath as any elected president would. Tyler, a former senator from Virginia, went on to defend the economic interests of wealthy Southern plantation owners, not the "log cabin and hard cider" voters who had put Harrison in office. He vetoed many bills, and was the first president ever to have a veto defeated by congressional override. Through no choice of his own, Tyler also established the precedent of only remaining in office through the current presidential term; though he tried to run for president, he was denied even a nomination. Americans had not found in Tyler what they had bargained for when they elected Harrison.

Between 1841 and 1963 every vice president who became president did so after the death of his predecessor. Each was recognized by both the government and the American public as the official President of the United States. However, the title of acting president also was used twice. It was first applied to Chester A. Arthur during the time between the shooting of President James A. Garfield and his actual death. Upon Garfield's death, Arthur assumed the title of president. When President Woodrow Wilson was seriously incapacitated for months, Vice President Thomas Marshall also chose to remain only acting president. The concern was that, if either Arthur or Marshall had in fact become president, the actual president at the time would have no apparent means by which to reclaim the office when and if they recovered from their disabilities. A constitutional crisis could have resulted. Instead, all branches of the government continued to use a loose interpretation of the ambiguously worded Article II, Section I of the Constitution to determine when and how to recognize vice presidents as presidents or acting presidents.

Though each accidental president did gain general acknowledgment as the president of the United States, it was never without some challenge from constitutional scholars. After the 1963 assassination of President John F. Kennedy and the consequent installation of Lyndon B. Johnson in the White House, American lawmakers were determined to clear up the confusion surrounding the presidential succession once and for all. The 25th Amendment, ratified in 1967 for this purpose, provides that the vice president should become president in the case of death or resignation, but should be only an acting president in the case of disability. Another clause of the 25th Amendment provides that in the case of a vacancy in the office of vice president, the president must appoint a successor. It was this provision that placed President Gerald Ford in office, along with vice president Nelson Rockefeller. When Agnew resigned as vice president, Nixon acted in accordance with the 25th Amendment and appointed Ford to fill the position. When Nixon resigned, Ford advanced to the presidency and appointed Rockefeller as vice president. Thus, the 25th amendment placed the nation under the administration of a president and vice president, neither of whom had been elected by American voters. The outcome of these events continue to spark debate over the constitutionality and practicality of the 25th Amendment.

To learn more about the 25th Amendment and how the presidential succession is determined, click here.