So do I; 1972 against McGovern. You can look it up. I still wasn't old enough to vote then. My first POTUS election was 1976.
The Teamsters backed Nixon’s *Phase II* as part of the deal to get Hoffa out of prison. After the AFL-CIO left the Labor Board (part of the Wage and Price Council) over the terms of *Phase II*, Nixon HAD to placate the Teamsters as they were the ONLY labor org. remaining on the Board. Without them, the Labor Board would have NO input from Labor and be shown publicly as the farce it was.
As for backing him against McGovern, that was just part of the split between the AFL-CIO and Teamsters. Had organized labor prevailed against the hard left of the RATs, Scoop Jackson would have been the RAT candidate and (imo) Nixon would have been defeated.