In Cold War travels, Bernie Sanders found much to admire. Now that’s a problem for his campaign.

The mayor of tiny Burlington, Vermont, was back from Nicaragua and eager to share the good news.

The country's Soviet-backed government - forged via armed rebellion - was cutting infant mortality, reducing illiteracy and redistributing land to peasant farmers. Its Sandinista leaders, branded terrorists by the U.S. government, impressed him with "their intelligence and their sincerity."

Three years later, Bernie Sanders was fresh off the plane from Moscow, reveling in the beauty of the land and the contentedness of the people.

And a year after that, he returned from Cuba having tapped into a revolutionary spirit "far deeper and more profound than I understood it to be."

With Sanders now surging to the top of the Democratic presidential field, those three-decade-old impressions introduced a volatile new element in the race Monday as rivals reacted to Sanders' decision to defend his remarks, not disclaim them.

Asked about his favorable reviews of Fidel Castro’s Cuba in a “60 Minutes” interview that aired on CBS on Sunday night, Sanders said the communist leader deserved criticism for “the authoritarian nature” of his government - as well as praise where it was due, including for “a massive literacy program.”

"Is that a bad thing? Even though Fidel Castro did it?" he asked.

The comments offered instant fodder for opponents who had already been sharing the old clips, and highlighted the risk to a candidate with a track record of sympathy for communist and socialist governments that is unlike any other recent Democratic nominee.

Rivals seized on the brand-new video to portray the senator from Vermont as naive - a possible preview of attack lines in Tuesday night's debate, and of the barrage Sanders is likely to endure in the general election if he makes it that far.

"Fidel Castro left a dark legacy of forced labor camps, religious repression, widespread poverty, firing squads, and the murder of thousands of his own people," former New York mayor Mike Bloomberg tweeted. "But sure, Bernie, let's talk about his literacy program."

Former South Bend, Indiana, mayor Pete Buttigieg compared Sanders to President Donald Trump, tweeting that after four years of giving dictators a pass, the United States needs "a president who will be extremely clear in standing against regimes that violate human rights abroad."

The fact that Sanders' long-ago travels in the communist world have become an issue in the 2020 campaign reflects how unorthodox a choice he would be to lead the Democratic Party.

Sanders has vowed to remake the party in his far-left image as a "democratic socialist," and he argues that his vision for a political revolution is best exemplified by thriving democratic, first-world societies such as Denmark.

Yet in the 1980s, during the dying days of the Cold War, Sanders indulged a fascination with far more disruptive and divisive strains of a socialist ideology he has embraced throughout his adult life.

Returning home from visits to some of the United States' most avowed enemies, Sanders offered some criticism but also praise in Vermont community television recordings. Many of the videos were kept in storage for decades - including during his 2016 campaign - and, even after being posted online, have remained relatively unknown.

Now, Sanders' comments are coming back to life as opponents say his warm feelings toward his hosts decades ago make him vulnerable to attack and reveal a soft spot for left-wing despots.

"If people are going to vote for socialist candidate Bernie Sanders, they need to understand what socialism means historically. And it's not Scandinavia," said Marion Smith, executive director of the congressionally authorized Victims of Communism Memorial Foundation.

Smith recently tweeted a clip from a 1988 news conference in which Sanders lauds the Soviet Union for its chandelier-filled transit stations and its "palaces of culture." Smith demanded an apology for what went unmentioned.

"He turned a blind eye to what was known about the ongoing systematic human rights abuses, the suppression of religious and ethnic minorities, the jailing of dissidents," Smith said. "He was very clearly joining the ranks of the useful idiots who believed in the propaganda of the Soviet Union and carried it to the West."

Sanders has consistently pushed back against accusations that he was duped, insisting his travels were about building bridges and avoiding conflict. "He believed then and believes now that issues of war and peace are local issues because they direct government investment away from working people here at home," the Sanders campaign said in a statement to The Washington Post. "That's why as mayor of Burlington he focused on Reagan's dirty wars."

Sanders is not the first would-be president to confront scrutiny over long-ago travels. When he ran in 1992, Bill Clinton faced questions over a 1969 trip to the Soviet Union. John Kerry, the Democrats' 2004 nominee, took heat from Republicans for a 1985 visit to Nicaragua - the same year that Sanders visited.

But Clinton was in Moscow as a student tourist, while Kerry went to Managua as a senator preparing to vote on whether to back President Ronald Reagan's plan to spend millions of dollars funding the ruling Sandinistas' rivals, the Contras. While there, Kerry challenged the government over its curbs on individual liberties, and he carried back to Washington a proposal for peace.

The reasons the mayor of Burlington, Vermont - population 38,000 - would repeatedly cross the world's great geopolitical chasm are less straightforward.

Sanders' infatuation with revolutionary left-wing movements, particularly those in Latin America, was long-standing, and it became a key feature of his first stint in elected office.

Sanders has recalled feeling "very excited" by Castro's 1959 revolution, which played out during his teens. "It just seemed right and appropriate that poor people were rising up against rather ugly rich people," he said in 1986.

As a college student at the University of Chicago, he was a member of the Young People's Socialist League. He spent his 20s and 30s as a radical activist, failed third-party candidate and sometime carpenter.

But it wasn't until Sanders was elected mayor of Burlington in 1980, at age 39, that he began putting his ideas into practice.

And while many of his local policies hewed to the conventional - redeveloping the city's waterfront or attracting a minor league baseball team - his forays into foreign policy stood out.

At the time, the Reagan administration was zealously fighting the Cold War. Democrats on Capitol Hill were following along in principle, if not always in the particulars. Sanders saw an opportunity to transform Burlington into a de facto capital for an alternative foreign policy, one that viewed the left-wing revolutions of Latin America less as threat than as opportunity.

In the early years of his mayoralty, that meant organizing a referendum disavowing U.S. support for the military regime in El Salvador and condemning the U.S. invasion of Grenada, which toppled a communist government.

Later - once he had proved himself an adept administrator of the city and was comfortably reelected - it meant travel to countries regarded as adversaries in Washington.

"He was trying to show that we weren't all Ronald Reagan," said Lauren-Glenn Davitian, a founder of CCTV, Burlington's public access television. "He was curious. And he was mad about how these stories were being told. He wanted to see for himself what was really happening."

When Sanders returned, he also wanted to share what he found with Burlington residents through interviews, news conferences and his own program on CCTV, "Bernie Speaks with the Community."

In July 1985, Sanders was back from Nicaragua having been, as he put it, "the highest-ranking American to visit them during the celebration of their revolution."

It had been six years since the Sandinistas stormed to power, overthrowing the hated Somoza dynasty, and the government had welcomed Sanders, as he described it, "in a special way." That meant access to the top leadership - right up to President Daniel Ortega.

As Sanders sat in a bare Burlington office recounting the trip - the sound of what appears to be a power drill blaring nearby - he acknowledged to his young CCTV interviewer that Sandinista-led Nicaragua was no panacea.

"You walk into houses and they're not houses like we know in Burlington, Vermont," Sanders said. "They are shacks."

Health care, he reported, was free. But it was "terrible. . . . It's not the Medical Center of Vermont there, believe me."

On the whole, however, he found plenty of reason for optimism. "Nobody denies that they are making significant progress," he asserted.

He was particularly impressed, he said, by the country's leaders, including Ortega, who is back in power in Nicaragua, having ruled for most of the past 40 years. Ortega now stands accused by human rights groups of violent oppression; in 1985, his grip on power was less absolute.

Watching the video decades later, Latin America scholar Richard Feinberg said Sanders appeared to have had "a rather simplistic" view of the developing world, as well as an overly rosy assessment of Nicaragua's leaders. But the reason for Sanders' attraction, he said, was clear.

"In Nicaragua he found his equivalents to his view of the United States . . . allegedly a small group of wealthy people at the top versus the struggling, noble poor," said Feinberg, who teaches at the University of California at San Diego and was a national security official during the Clinton administration.

Sanders' visit to the Soviet Union in the late spring of 1988 offered a more complex picture. The CCTV video marking the trip - which was posted online just last year - begins in an unlikely place: Sanders' sun-splashed wedding on the shores of Lake Champlain. The next day, the newlyweds were off on what Sanders has described as "a very strange honeymoon."

The trip was actually official city business, with Sanders and his delegation exploring the potential for a sister-city relationship with the Soviet city of Yaroslavl.

The Soviet Union was in the midst of a momentous transition - one that would end with its collapse a little over three years later.

Speaking to reporters at the airport upon his return, Sanders lauded "the openness of Soviet officials to acknowledge many of the problems that they have. I went there expecting them to say 'Everything is great, there's no problem.' That certainly wasn't the case."

As in Nicaragua, health care was lacking, even if it was free. "They acknowledge it's 10 years behind the U.S. in terms of medical technology," he said.

But Sanders also found much to admire, including investments in culture and "the most effective mass transit system that I've ever seen."

So did his new wife, Jane, who was then director of Burlington's youth services department and who marveled at the fact that resources weren't concentrated in private hands. "They put the money into public facilities," she told reporters at a news conference.

Bernie Sanders expressed hope that, after "a dismal history," the Soviet Union could be redeemed by moving "forward into some of the early visions of their revolution, what their revolution was about in 1917."

William Taubman, an Amherst College historian of the Soviet Union who was living there at the time, said Sanders' comments need to be understood in the context of the moment, which was dominated by then-leader Mikhail Gorbachev's policy of opening and liberalizing his nation.

"He was not doing what the real suckers might have done, which was to say, 'Gosh isn't it wonderful?' " Taubman said. "I don't think he was a dupe."

A year later, Sanders was seemingly more impressed by his experience in Cuba. By that time, he was on his way out as mayor and was no longer producing new episodes of his community television program.

But he shared his thoughts with the Burlington Free Press, telling the paper that while the country had "deficiencies," it appeared to have solved the sort of problems that still plagued the far wealthier United States.

"I did not see a hungry child. I did not see any homeless people," he said. "Cuba today not only has free health care but very high quality health care."

He had wanted to meet Castro. He had to settle for the mayor of Havana. But the people's enthusiasm for their leader told him what he needed to know.

“The people we met,” he reported “had an almost religious affection.”