From the BBC: The Lyndon Johnson Tapes: Richard Nixon's 'Treason'

This, if true, and it is at least partially backed up by tapes, is scandalous: according to the BBC (not always the most reliable source) during the 1968 U.S. presidential campaign Richard Nixon, then a candidate in a very tight race with Hubert Humphrey, scuttled the Paris peace talks the U.S. was engaged in with the North Vietnamese. According to the BBC Nixon used Anna Chennault (born Chen Xiangmei (陳香梅), she was a Republican operative who had a very interesting life) to convince the South Vietnamese to reject an agreement the U.S. and the North Vietnamese had worked out because Nixon claimed the South Vietnamese would be better treated by his administration. Of course, they ultimately were not: South Vietnam fell to the North in 1975 after the additional deaths of more than 100,000 people. If the talks had been successful before the 1968 election it seems likely that the democratic candidate would have one.

And as to who that candidate would be, there's another significant historical note in the BBC story: President Lyndon Johnson wanted to go to the convention and re-enter the race and get the nomination himself. He didn't for several reasons. One, apparently, was that he then knew Nixon had prevented the peace accord, a fact he learned through NSA and other surveillance of Chennault and the ambassador, among others. Johnson called Nixon a traitor and said he had blood on his hands.

The claims in the BBC article have been in the public arena for several years, though they've received scant attention. Both Johnson and his Secretary of Defense, Clark Clifford, claimed it was true and not based on speculation. Numerous writers have claimed it was the case, and it seems to be supported (if by innuendo) by the former South Vietnamese ambassador's own memoir.

Via Kottke.