“Mayor Sanders was proud to join dozens of American cities in seeking to end the Cold War through a Sister Cities program that was encouraged by President Reagan himself,” a Sanders campaign spokesman, Mike Casca, said in a statement. “The exchange between Burlington and Yaroslavl, which continues to this day, confirmed Sanders’s long held view: by meeting face to face, we can break down the barriers and stereotypes that exist between people and their governments.”

Mr. Sanders’s involvement in the Cold War debate grew in the 1980s as he forcefully opposed the Reagan administration’s plans to have Burlington and other American cities make evacuation plans for a potential nuclear war.

Instead, Mr. Sanders reached out to the Soviet Union via an organization based in Virginia, requesting a sister-city partnership with the Cold War adversary in an effort to end the threat of nuclear annihilation.

“We were saying: The goal is to not have a nuclear war, not to plan and prepare for it,” said Terry Bouricius, a Burlington alderman at the time who accompanied Mr. Sanders on the trip.

This was also the era of perestroika and glasnost, economic and cultural changes promoted by Mr. Gorbachev that had sparked optimism among some in the West — along with skepticism from many others — that the Kremlin was prepared to adopt a more conciliatory stance abroad and provide greater freedoms to its own people.

While Mr. Sanders has taken heat from President Trump and his campaign for this outreach to the Soviets, his supporters say it was a timely effort to help defuse tensions and stands in contrast to Mr. Trump’s affinity for strongman leaders like Russia’s current president, Vladimir V. Putin.

Proposing a Partnership

In December 1987, the records show, Mr. Sanders spoke by phone with Yuri Menshikov, the secretary of the Soviet sister-city organization in Moscow. In a follow-up letter later that month, Mr. Sanders said he had received word that Yaroslavl would be an ideal partner. He proposed leading a Burlington delegation to Yaroslavl to lay the foundation for a sister-city relationship.