In 1951, the U.S. ratified the 22nd Amendment, making a president ineligible to be elected a third time, or to be elected a second time after also having succeeded to the presidency and served more than two years. The latter clause would have applied to Truman in 1952, but he was still eligible to run for a third term since a grandfather clause in the amendment explicitly excluded the current president from its provisions.
At the time of the 1952 New Hampshire primary, no candidate had won Truman's backing. His first choice, Chief Justice Fred Vinson said no; Illinois Governor Adlai Stevenson repeatedly said no; Vice President Barkley was considered too old; and Truman distrusted and disliked Senator Estes Kefauver, whom he privately called "Cowfever."<43>
Truman's name was on the New Hampshire primary ballot, but Kefauver won, so Truman announced his decision not to run on March 29.<44> Stevenson, having reconsidered his presidential ambitions, received Truman's backing and won the Democratic nomination. Eisenhower crusaded against what he denounced as Truman's failures regarding "Korea, Communism and Corruption" -- and won in a landslide.
http://en.wikipedia.org/wiki/Harry_Truman#1952_election