He was tall and sometimes goofy, known for his ability to mimic a goose’s honk. He spoke in long, meandering paragraphs, musing on Franklin D. Roosevelt, Karl Marx and Bob Marley. He took painstaking notes on encounters with farmers, doctors and revolutionary fighters.

Bill de Blasio , then 26, went to Nicaragua to help distribute food and medicine in the middle of a war between left and right. But he returned with something else entirely: a vision of the possibilities of an unfettered leftist government.

As he seeks to become the next mayor of New York City, Mr. de Blasio, the city’s public advocate, has spoken only occasionally about his time as a fresh-faced idealist who opposed foreign wars, missile defense systems and apartheid in the late 1980s and early 1990s. References to his early activism have been omitted from his campaign Web site.

But a review of hundreds of pages of records and more than two dozen interviews suggest his time as a young activist was more influential in shaping his ideology than previously known, and far more political than typical humanitarian work.

Mr. de Blasio, who studied Latin American politics at Columbia and was conversational in Spanish, grew to be an admirer of Nicaragua’s ruling Sandinista party, thrusting himself into one of the most polarizing issues in American politics at the time. The Reagan administration denounced the Sandinistas as tyrannical and Communist , while their liberal backers argued that after years of dictatorship, they were building a free society with broad access to education, land and health care.

Today, Mr. de Blasio is critical of the Sandinistas’ crackdown on dissenters, but said he learned from his time trying to help the Central American country.

“My work was based on trying to create a more fair and inclusive world,” he said in a recent interview. “I have an activist’s desire to improve people’s lives.”

Mr. de Blasio became an ardent supporter of the Nicaraguan revolutionaries. He helped raise funds for the Sandinistas in New York and subscribed to the party’s newspaper, Barricada, or Barricade. When he was asked at a meeting in 1990 about his goals for society, he said he was an advocate of “democratic socialism.”

Now, Mr. de Blasio, a Democrat, describes himself as a progressive. He has campaigned for mayor as a liberal firebrand who would set out to reduce inequality in the city by offering more help to poor families and asking wealthy residents to pay more in taxes. He said that seeing the efforts of the Sandinistas up close strengthened his view that government should protect and enhance the lives of the poor.

“It was very affecting for me,” Mr. de Blasio said of his work with Nicaraguans, in a recent interview. “They were in their own humble way, in this small country, trying to figure out what would work better.”

An Epiphany Abroad

The roots of Mr. de Blasio’s progressive brand of politics lie in the shadows of volcanoes, thousands of miles from the city he now hopes to lead, at a decaying health clinic in Masaya, a small Nicaraguan city.

Mr. de Blasio, bearded, gawky and cerebral, had arrived in the city as part of a 10-day tour of Nicaragua in 1988, the capstone of the year he spent as an employee of the Quixote Center , a social justice group in Maryland.

The center, founded by Catholic leaders, officially did not take sides in the Nicaraguan dispute, though much of its aid went to help families sympathetic to the Sandinistas. And its work was intensely political. One of the center’s leaders once likened American efforts in Nicaragua to a “policy of terrorism,” and its harshest critics accused it of hewing to a Marxist agenda. In the mid-1980s, the Treasury Department investigated whether the center had helped smuggle guns, but the claim was never substantiated, and the group’s leaders said the inquiry was politically motivated.

At the time, gunshots and protest songs permeated the Nicaraguan air as the Sandinistas waged war with the contras, a counterrevolutionary movement backed by the United States. The Sandinista slogan declared, “Free homeland or death!”

American leaders feared that the Sandinistas, who received weapons from the Soviet Union and supplies from Cuba, would set off a socialist movement across Latin America. But the United States’ decision to intervene in Nicaragua was unpopular, especially after it was revealed that the Reagan administration had covertly financed the contra rebellion, even after Congress had voted to cut off assistance to the fighters.

The involvement of the United States galvanized activists across the country who saw parallels to Vietnam. Tens of thousands of Americans ? medical workers, religious volunteers, antiwar activists ? flocked to Nicaragua hoping to offset the effects of an economic embargo imposed by the United States. Many were drawn to the idea of creating a new, more egalitarian society. Critics, however, said they were gullible and had romanticized their mission ? more interested in undermining the efforts of the Reagan administration than helping the poor.