"...It begins in the summer of 1968. Nixon feared a breakthrough at the Paris Peace talks designed to find a negotiated settlement to the Vietnam war, and he knew this would derail his campaign.From Marilyn Young's The Vietnam Wars 1945-1990:
He therefore set up a clandestine back-channel involving Anna Chennault, a senior campaign adviser.
At a July meeting in Nixon's New York apartment, the South Vietnamese ambassador was told Chennault represented Nixon and spoke for the campaign. If any message needed to be passed to the South Vietnamese president, Nguyen Van Thieu, it would come via Chennault.
In late October 1968 there were major concessions from Hanoi which promised to allow meaningful talks to get underway in Paris - concessions that would justify Johnson calling for a complete bombing halt of North Vietnam. This was exactly what Nixon feared.
The Paris peace talks may have ended years earlier, if it had not been for Nixon's subterfuge. Chennault was despatched to the South Vietnamese embassy with a clear message: the South Vietnamese government should withdraw from the talks, refuse to deal with Johnson, and if Nixon was elected, they would get a much better deal.
So on the eve of his planned announcement of a halt to the bombing, Johnson learned the South Vietnamese were pulling out.
He was also told why. The FBI had bugged the ambassador's phone and a transcripts of Anna Chennault's calls were sent to the White House. In one conversation she tells the ambassador to "just hang on through election".