President Lyndon B. Johnson prepares for the 1966 State of the Union address with, from left, former aide Richard Goodwin, Jack Valenti and Joseph A. Califano Jr. (The White House/AP)

On the eve of the midterm elections, Americans are nervous about foreign interference, especially since we still have not resolved just how much Russia interfered in the 2016 elections and whether they colluded with President Trump’s campaign. Amazingly, however, whatever special counsel Robert S. Mueller III finds, the Russian interference in 2016 won’t be the most jaw-dropping case of foreign involvement in an American election.

The reason: In 1968, Richard Nixon sought help from South Vietnam to defeat Democratic Vice President Hubert Humphrey. And when President Lyndon B. Johnson learned of this “treason,” he did nothing to reveal or halt it — because he wanted his own vice president to lose.

Johnson’s animosity toward Hubert Humphrey revealed itself as soon as the latter started his term as vice president. In March 1965, the newly sworn-in vice president warned Johnson against expanding the Vietnam conflict. In a memo, he argued that the war was unwinnable militarily and that the public, especially the Great Society political coalition, would not support a war based on a vague “national security” claim for long. Humphrey held that the best time to settle was now, following Johnson’s landslide victory in November 1964, which included a pledge of no wider war.

Humphrey’s memo proved prescient, but it infuriated the president, who in near-paranoid fashion presumed his vice president had written it to leak later, in the event he needed political cover. Between 1965 and 1967, Johnson exerted great pressure on Humphrey to support his expansion. Ever loyal, Humphrey became a spokesman for the war as a means of halting North Vietnam’s aggression and bringing Great Society measures to South Vietnam.

In 1968, growing opposition to the war forced Johnson to the sidelines. But the president remained determined to shape the conversation about Vietnam. He threatened to “destroy” Humphrey, then heading toward securing the Democratic nomination to succeed Johnson, if he parted from administration policy by proposing a bombing halt intended to spur peace talks.

The president also accepted an offer from Nixon: Nixon would not criticize Johnson’s Vietnam policy if Johnson held to his firm position and did not declare a bombing halt, which would have bolstered Humphrey’s campaign. The deal prompted Defense Secretary Clark Clifford to fret that Nixon had maneuvered Johnson into a hard-line policy intended to “freeze poor Hubert out in the cold.”

At the Democratic convention in August, Johnson continued to apply pressure. He and his allies overrode a Humphrey-supported platform plank calling for a bombing halt with one that put off a stoppage until the war was virtually over. Humphrey left the convention deeply despaired and uncertain whether Johnson favored “Nixon or me.” Meanwhile, Clifford wondered “if in his heart of hearts Lyndon Johnson really wanted Humphrey to win.”

Nixon’s efforts to win over Johnson also intensified. The Rev. Billy Graham reassured the president that if elected, Nixon would never blame Johnson for the war and would honor him in his post-presidential years.

This flattery worked. Johnson actively continued to hurt his vice president’s campaign. First, he denied Humphrey’s cash-starved operation funds from the Democratic National Committee and his base of wealthy Texas donors and refused to campaign for him in the border states where he might have helped. Johnson also denied Humphrey his statutory National Security Council seat and access to key war information, and refused to reveal publicly that the Nixon campaign had received $500,000 from the military junta in Greece that had overthrown the government in Athens in 1967. Johnson also began to curse out Humphrey regularly in front of White House staff, leading trusted aide Charles Murphy to believe the president preferred Nixon to be elected.

By late September, Humphrey’s campaign was in dire straits. Desperation prompted Humphrey to take a bold gambit. On Sept. 30, he declared that as president he would propose a bombing halt as an acceptable “risk for peace.” By separating himself from Johnson’s war policy, Humphrey breathed new life into his campaign.

But he also angered the president. Johnson told two close supporters that Nixon “is following my policies more closely than Humphrey.” When Hanoi indicated an Oct. 31 bombing halt would bring negotiations on Nov. 2, Johnson denounced this as a “dangerous” date that would be seen as helping Humphrey in the Nov. 5 election.

Even more critically, when Johnson learned on Oct. 28 from FBI, CIA and NSA wiretaps that Nixon’s campaign had been working through prominent Republican fundraiser Anna Chennault and South Vietnam’s Ambassador Bui Diem to get the Saigon government (and even Hanoi) to reject peace talks to get a better deal from a President Nixon, Johnson merely called Republican Sen. Everett Dirksen, a Nixon ally, to say the candidate should stop playing “dirty pool.”

Then, as Johnson finally prepared to announce a bombing halt on Oct. 31 — to begin the day after the election — he called Humphrey to insist that he should let “all the laurels come to me.” Johnson also told Humphrey that Nixon was encouraging South Vietnam to resist talks to ensure the Humphrey’s defeat, but he insisted that he had no “hard proof” of the Republican candidate’s action to impede peace efforts. Humphrey understood this meant Johnson would not give him access to the tapes he needed to charge Nixon publicly with this perfidy.

When Johnson learned that Chennault had called Diem to say that her “boss” (Nixon’s running mate, Spiro Agnew) had called to urge the South Vietnamese to “hold on” because “we’re gonna win,” Johnson called Dirksen again to fume, “This is treason.” But he did nothing about it, opting instead to rant again about Humphrey’s Sept. 30 bombing halt speech.

Johnson betrayed his vice president, and all Democrats, by doing nothing except undermine Humphrey, who lost the popular vote by less than three-fourths of 1 percent, or roughly 500,000 votes. Standard White House support might have helped Humphrey win tight races in at least two of three usually Democratic states: New Jersey, Pennsylvania and Illinois, with a total of 69 electors. That would have left Nixon short of the required 270 electoral votes necessary to secure the presidency and sent the election into the Democratic-controlled House of Representatives. Because it is uncertain how southerners, especially from the five Deep South states George Wallace captured, would have voted, it is impossible to know whom the House would have selected as president.

Regardless, Johnson’s betrayal of Humphrey and the Democratic Party opened the way to Nixon’s election and growing Republican use of the “Southern strategy” of heating race relations and xenophobia to win elections. Ironically, Johnson also spurred the now half-century Republican effort to dismantle his beloved Great Society. In many ways it is these practices on trial this week as Americans head to the polls.