The current uproar over the Russian hacking into the 2016 presidential election is not the first, nor the most consequential, foreign intrusion into American politics.

In the 1968 election, agents of Richard Nixon, with his knowledge and acquiescence, encouraged South Vietnam leaders to boycott Paris peace talks with the North Vietnamese. They promised that the Saigon regime would get a better deal from President Nixon than from his Democratic rival, Hubert Humphrey. The South Vietnamese stayed away and Nixon was narrowly elected.

This deal, which retiring President Lyndon Johnson called an act of “treason,” violated the Logan Act barring such intervention in foreign policy. A file called the “X” envelope, compiled four years later by LBJ adviser Walt Rostow, offered more evidence that the Nixon team had repeatedly pressured South Vietnamese President Nguyen Van Thieu to boycott the talks, and this did help Nixon win.

Johnson in 1968 had ordered the FBI to tail and tape record a visit to the Saigon embassy in Washington of Nixon fund-raiser Anna Chennault, who conveyed the message from the Nixon campaign. LBJ gave the evidence to Humphrey, who inexplicably declined to make it public, to Johnson’s bitter disappointment.

The Nixon effort to sabotage the peace talks and thus frustrate Johnson’s 11th-hour attempt to help Humphrey’s election chances, is supported in a recent New York Times article by John A. Farrell, author of a new Nixon biography. It reproduces handwritten notes by Nixon Chief of Staff H.R. Haldeman of an Oval Office meeting reporting Nixon’s instructions to pressure Thieu and to use “any other way to monkey wrench” the peace talks.

Humphrey’s reason for not making the Nixon intervention public was that it would have required his own illegal use of classified intelligence. Johnson was so angry, according to aide Joe Califano, that it caused “a lasting rift” between LBJ and HHH.

Humphrey in his later memoir wrote: “I wonder if I should have blown the whistle on Anna Chennault and Nixon. He must have known about her call to Thieu. I wish I could have been sure. Damn Thieu. Dragging his feet this past weekend hurt us. I wonder if that call did it. If Nixon knew. Maybe I should have blasted him anyway.”

The very notion that Nixon was incapable of such a scheme is mind-boggling, considering Humphrey’s awareness of Nixon’s record of political dirty tricks. Had the plot been discovered by Election Day, there may never have been a Nixon presidency or, for that matter, the Watergate scandal that led to his 1974 resignation.

As late as June 17, 1971, Nixon remained worried that his efforts to scuttle the 1968 peace talks would surface. Told by one of his sleuths that a file on the caper was at the Brookings Institution, he ordered that it be broken into. Nixon replied on one tape: “God damn it, get in and get those files. Blow the safe and get it.” And again on June 30: “You talk to Hunt,” he said, referring to E. Howard Hunt, one of the 1972 Watergate break-in artists. “I want the break-in. Hell, they do that. You’re to break into the place, rifle the files and bring them in.”

The Brookings break-in never happened, but Nixon’s relentless demand for it demonstrated his continuing fear that his plot to sabotage the peace talks could be revealed. A diligent Nixon researcher at the University of Virginia, Ken Hughes, observed in his 2014 book, “Chasing Shadows,” that Brookings was the missing link in the whole Watergate saga.

Rostow originally honored an instruction from LBJ that a 50-year embargo be placed on publication of the X-file. He turned it over to the LBJ Library, thus sealing the strong evidence that Nixon tried to kill the Paris talks. Later, Rostow seemed to regret his decision, writing that Nixon “got away with it” in 1968. But as Hughes noted, it was Rostow’s decision that assured the ultimate Nixon dirty trick “didn’t see the light of day” in time to hurt him.

In 1994, after the deaths of Johnson and Nixon, the LBJ Librarian opened the X-file, revealing the scheme that, if exposed earlier, could have brought Nixon’s impeachment and/or resignation.

Jules Witcover’s latest book is “The American Vice Presidency: From Irrelevance to Power.” He wrote this for the Tribune News Service.