Twenty-second Amendment to the United States Constitution
Since coming into force in 1951, the amendment has applied to six presidents who have been elected twice: Dwight D. Eisenhower, Richard Nixon, Ronald Reagan, Bill Clinton, George W. Bush, and Barack Obama. It has also applied to two who entered office intra-term due to their predecessor's death or resignation: Lyndon B. Johnson and Gerald Ford. Johnson became president in November 1963, following the assassination of John F. Kennedy, served out the final of Kennedy's term, and was elected to a full four-year term in 1964. Four years later, he briefly ran for a second full term, but withdrew from the race during the party primaries. Had Johnson served a second full term – through January 20, 1973 – the total length of his presidency would have been ; as it happened, Johnson died two days after this date. Gerald Ford, who became president in August 1974 following the resignation of Richard Nixon, served the final of Nixon's term, and attempted to win a full four-year term in 1976, but was defeated by Jimmy Carter. Johnson was eligible to be elected to two full terms in his own right, as he had served less than two years of Kennedy's unexpired term, whereas Ford was eligible to be elected to only one full term, as he had served more than two years of Nixon's unexpired term.
Lyndon B. Johnson
As he had served less than 24 months of President Kennedy's term, Johnson was constitutionally permitted to run for a second full term in the 1968 presidential election under the provisions of the 22nd Amendment.
Initially, no prominent Democratic candidate was prepared to run against a sitting president of the Democratic Party. Only Senator Eugene McCarthy of Minnesota challenged Johnson as an anti-war candidate in the New Hampshire primary, hoping to pressure the Democrats to oppose the Vietnam War. On March 12, McCarthy won 42 percent of the primary vote to Johnson's 49 percent, an amazingly strong showing for such a challenger. Four days later, Senator Robert F. Kennedy of New York entered the race. Internal polling by Johnson's campaign in Wisconsin, the next state to hold a primary election, showed the President trailing badly. Johnson did not leave the White House to campaign.