James Riddle "Jimmy" Hoffa was an American labor union leader. Hoffa was closely involved with the International Brotherhood of Teamsters union, as an organizer, from 1932 to 1975. By 1952, Hoffa had risen to national vice-president of the IBT, and served as the union's General President between 1958 and 1971. He secured the first national agreement for teamsters' rates in 1964, and played a major role in the growth and development of the union, which eventually became the largest single union in the United States, with over 1.5 million members at certain times, during his terms as its leader. Hoffa became involved with organized crime from the early years of his Teamsters work, and this connection continued until his disappearance in 1975. He was convicted of jury tampering, attempted bribery, and fraud in 1964. Hoffa was imprisoned in 1967, and sentenced to 13 years, after exhausting the appeal process. In mid-1971 he resigned the Teamsters' presidency, an action that was part of a pardon agreement with President Richard Nixon, to facilitate his release later that year. Nixon blocked Hoffa from union activities until 1980. Hoffa attempted to overturn this order and to regain support.