On December 24, 1971, President Richard Nixon pardoned James "Jimmy" Hoffa, former Teamsters President, on the condition that he no longer participate in Teamsters union activities. The deal set in motion large, clandestine cash payments from Teamsters President Frank Fitzsimmons to the Nixon reelection campaign in 1972, and a later payment that some speculate went to help pay for Nixon's failed coverup of the Watergate break-in. It also helped Nixon gain the Teamsters endorsement at a time when most labor unions considered it impossible to endorse a Republican even if individual labor leaders were willing to do so.
Speculation about why Nixon pardoned Hoffa has persisted for decades, partly as a result of conspiracy theories about Nixon's ties to organized crime. No doubt it was in exchange for Teamster support for his reelection-- whether that support came in the form of endorsements, money, and even "dirty tricks" (Nixon tapes suggest that Nixon used Teamsters "thugs" and "murderers" to assualt anti-war protesters, and biographer Anthony Summers has speculated that the Teamsters had a role in assaulting Abbie Hoffman at Nixon's request).
Now, a new document* that I found in the Nixon archives suggests one additional reason: Charles Colson, Nixon's aide, needed a way to get the Teamsters to not re-join the AFL-CIO before the 1972 election. Colson worried that if the Teamsters, which had split from the AFL-CIO in 1968, re-joined the labor federation, that it would not be able to "quietly work very hard for us, with money and organizational support... it is in our interest to see that the merger does not take place." In another document**, Colson called a potential merger "very much against our interests politically." Colson was the main person in the Nixon administration who facilitated Hoffa's pardon. After being pushed out of the White House for being too close to the Watergate break-in conspirators, Colson went on the Teamsters payroll through his DC law firm. He then went to jail for his role in the Watergate scandal, where he was "born again" as an evangelical Christian.
Hoffa was killed when he tried to get back in union politics, which would have violated the terms of his pardon. Fitzsimmons didn't resign until 1981, and the Teamsters didn't re-affiliate with the AFL-CIO until 1987.
*Box 4, Folder 28, Contested Materials: White House Special Files, Nixon Library-Yorba Linda, National Archives and Records Administration
**Box 3, Folder 12, Contested Materials: White House Special Files, Nixon Library-Yorba Linda, National Archives and Records Administration