Last week, a new wrinkle was added to what has been the most unlikely presidential campaign in modern American history: The Russian government is actively seeking to tip the balance for Donald Trump.

With word that Russian intelligence is likely responsible for the hacking and release of emails from the Democratic National Committee, it represents a blockbuster story of foreign involvement in a presidential campaign. But it’s not unprecedented.

Indeed, 48 years ago, during the 1968 presidential campaign, the Soviet Union embarked on its own campaign to help defeat Richard Nixon and elect Hubert Humphrey president. Amazingly, they almost succeeded.

While Soviet leaders had long had preferences in US presidential elections, rarely did they try to affect the outcome. But in 1968, armed with not-so-fond memories of Nixon’s red-baiting in the 1950s and his generally hawkish stance toward the Soviet Union, they feared that a Republican win in November would mean, potentially escalation in Vietnam, and a return to heightened Cold War tensions.

Hubert Humphrey, the Democratic candidate, also supported the war in Vietnam. The Democratic candidate also had a reputation as a Cold War hawk—but the Soviets preferred dealing with Democratic presidents. They believed Humphrey would be more likely to end the war in Vietnam than Nixon.

But bringing an end to that war was easier said than done. The Soviet’s ally in North Vietnam and America’s ally in the South would have to agree. By the fall of 1968, the American military involvement in Vietnam was in its third year; and while the United States bore the greatest burden, the Soviets saw the continuation of the conflict as a drain on their own resources— and a stumbling block to better relations with the Americans.

Moscow viewed an end to the war in Vietnam as not only desirable for Soviet interests, but as a way to help Humphrey win the election. A light at the end of the tunnel on Vietnam might be enough to convince the American people to give the Democrats four more years.

But by the fall of 1968, there was little reason for such optimism. Beginning earlier that spring, U.S. and North Vietnamese negotiators had begun meeting in Paris in hoping of reaching a political settlement to the conflict. But the talks went nowhere, largely because neither side showed any willingness to compromise on their core positions. The United States wanted to the North to stanch the flow of Vietcong rebels into the South, stop shelling South Vietnamese cities and let the South Vietnamese join peace talks. The North wanted an unconditional U.S. bombing halt. Talks dragged on through the spring and summer with little indication of a possible breakthrough.

It was the Soviets, however, who helped to prime the pump. According to one historian of Soviet engagement in Vietnam, Moscow “was involved at every stage in resolving the problems that threatened to delay and even to disrupt a settlement,” while at the same time successfully limiting public discussion of their involvement in the talks.

They instructed their diplomats in Paris, Washington and Vietnam to push for a compromise. Behind the scenes they served as a conduit for messages between the two sides, passing message to the White House as early as the summer that if the U.S. unconditionally declared a bombing halt that it could lead to concessions from the North and “create a turning point at the meetings in Paris.”

White House officials did their part as well— reminding Soviet diplomats that it was unlikely “they would do any better in negotiations with President Johnson’s successor, whoever he might be.”

Finally, in early October, the long-sought breakthrough was achieved. The North Vietnamese indicated they would concede to South Vietnam’s presence at talks if the American stopped bombing the North. The United States got about 90 percent of what they were demanding from the North and on October 31 Johnson announced the bombing halt over North Vietnam. Peace, it seemed, might be at hand.

But it was not to be. For months before Johnson’s speech, Nixon campaign officials (and possibly the candidate himself) had been in contact with a prominent GOP donor, Anna Chennault, who served as a conduit to the South Vietnamese. Nixon staffers got her to lean on Saigon to stay away from the Paris talks, convincing them that under a Nixon presidency, the South would get a better deal.

While South Vietnamese leaders had plenty of reason to already believe this— and certainly preferred the hawkish Nixon over Humphrey—Chennault’s intervention was an unprecedented involvement of a presidential campaign in diplomatic negotiations. Johnson would call the Nixon campaign’s intervention tantamount to “treason.”

There’s no way to say for certain whether a last minute deal with South Vietnamese acquiescence would have won the election for Humphrey. But when one considers that he lost by only half-a-million million votes, it’s hard not to ask “what if?”

Still for the intrigue around the Paris talks, the Soviet involvement in the ’68 election was not just restricted to diplomacy. The U.S. Ambassador to the Soviet Union, Anatoly Dobrynin, was instructed by the Soviet Foreign Ministry to do everything in his power to help Humphrey, including financial assistance.

While Dobrynin initially resisted the request – fearing a backlash if it was ever revealed – he nonetheless played the loyal solder. At a breakfast meeting with Humphrey he brought up the subject of the Democrat’s campaign financing and implicitly suggested that the Soviets could be helpful. Humphrey quickly caught wind of where Dobrynin was going and quickly cut him off. He told the Ambassador it was more than enough to have Moscow’s good wishes – and not their cash. Writing in his memoirs, the relieved Dobrynin said “it was the only time Moscow tried to intervene directly to help a favorite candidate – and it got nowhere.”

Fifty years later, however, the Republican nominee for president seems slightly more inclined to accept Moscow’s assistance in his presidential campaign. Unfortunately, for him and the revelations that emerged last month, Russian involvement in American political affairs is handled a lot less subtly than was the case back in 1968.