Nixon (or at least his campaign chief of staff) contacted South Vietnam and promised them a better deal with him as President. LBJ knew this, because he was surveilling the South Vietnamese embassy in the US. There are tapes of him denouncing Nixon for "treason," and he had the Nixon campaign planes bugged.
It is undeniable that Nixon violated the Logan Act which forbids unauthorized parties from interfering with governmental negotiations.
After the election, neither side wanted to push it, though: Nixon didn't want it to come out that he had extended the war, and LBJ didn't want it to come out that he had bugged a political opponent's plane.
Ted Kennedy's intention to visit Beijing to gain advantage 1972 election, was "leaked" by Beijing to McNamara in 1971. This resulted in Nixon walked publicly, in daylight, into the Tiger's den, and in turn caused the landslide defeat of McGovern.
The one who had secretly contacted Beijing was McGovern himself.
With Iran, Reagan cause the Contragate affair.