What factor led to Nixon's re-election in 1972

I would answer: Nixon's appeal to the "Silent Majority" in America gave him sufficient support to win re-election in 1972.

Context/detail:

Richard Nixon sought to appeal to what he called "The Silent Majority" of American citizens - - the everyday, middle-class, working folks who were not part of the anti-war protests that had been happening in the country. On November 3, 1969, during his first year in office, President Nixon called on that "silent majority" in a major radio and television address. In response to continuing war protests, Nixon urged solidarity in support of the war effort in Vietnam War effort, saying that the United States was "going to keep our commitment in Vietnam."

Nixon's efforts in 1969 did not stop the war protests. (The folks protesting weren't part of his "Silent Majority" demographic). However, the 1972 election tended to vindicate Nixon and his appeal to average working-class voters. He won a landslide victory, carrying 49 of 50 states.

Other factors that worked in Nixon's favor were the pursuance of détente with the Soviet Union and his opening of relations with the People's Republic of China, which began with his trip there in February, 1972.