Tough to say how Nixon could win in 1972. After losing in both 1960 and 1968, Nixon got the GOP nomination because no one else wanted it (neither did Nixon). Since 1932 the only President the Republicans could elect was Eisenhower, with Nixon coming closer than anyone else, and everyone assumed that we would be in a period of one party Democratic governance at the federal level for the forseable future.
If the Republicans had had a chance of winning in 1972, they would have nominated someone other than Nixon. With a more viable Republican Party, in fact most likely Nixon would have won in either 1960 or 1968.
I mean if Lyndon Johnson had been able to serve out his term and not had his heart attack in 1972, then McGovern wouldn't have had the incumbency advantage, but then someone other than Nixon would have been the GOP nominee. Or is the POD Nixon winning in 1968? That one has been done lots of times on this board.
So the POD is either Nixon winning in 1968 or the Democrats are in a weaker position in 1972 and some alternative Republican manages to win that year. Obviously things change depending on who exactly gets elected instead in 1972.