John J. Sweeney, a New York union researcher who climbed to the pinnacle of the American labor movement in the 1990s, leading the A.F.L.-C.I.O. for 14 years through an era of fading union membership but rising political influence, died on Monday at his home in Bethesda, Md. He was 86.

Carolyn Bobb, an A.F.L.-C.I.O. spokeswoman, confirmed the death. She did not specify the cause.

As president, from 1995 to 2009, of the nation’s largest labor federation — 56 unions with 10 million members near the end of his tenure — Mr. Sweeney flexed labor’s political muscle with thousands of volunteers and helped elect Barack Obama to the presidency in 2008. Over the years, he also helped elect Democrats to seats in Congress, to governorships and to state legislatures across the country.

His tougher task, a quest to reinvigorate and diversify the faltering labor movement itself, had the weight of history pushing against him.

For decades in the 20th century, labor had not welcomed women, African-Americans, Latinos or Asian-Americans, often engaging in blatantly discriminatory tactics to preserve the dominance of white men in the workplace. Substantial but uneven gains had been achieved since the civil rights era of the 1960s, when unions began removing “whites only” clauses from their constitutions and bylaws.

But Mr. Sweeney, still facing lopsided demographics, plotted a sea change. He crusaded to bring women and minorities into the fold, often in leadership posts; made alliances with civil rights groups, students, college professors and the clergy; and championed low-wage workers, shifting away from the A.F.L.-C.I.O.’s traditional emphasis on protecting the best-paid union jobs.

In Mr. Sweeney’s campaign for the federation presidency, his running mate, for the newly created post of executive vice president, was Linda Chavez-Thompson, a Texas sharecropper’s daughter. She was the first minority group member ever elected to organized labor’s top executive ranks.

The 1995 balloting itself was unique: It was the first contested election in the history of the federation, which had been created in 1955 by a merger of the American Federation of Labor and the Congress of Industrial Organizations after a long estrangement.

A signature Sweeney initiative encouraged the recruitment of thousands of immigrants to his unions. Many members had long been hostile to undocumented workers, accusing them of stealing union jobs and dragging down wage scales. Mr. Sweeney rebuked such talk as discriminatory and called for justice that included better treatment for underpaid immigrants and a path to citizenship for those in the United States illegally.

Critics contended that Mr. Sweeney’s policies were locked in a liberal past, deploying mid-20th century civil rights and blue-collar union strategies to organize 21st century workers with internet skills. Mr. Sweeney rejected that claim, just as he had rebuffed corporations that moved jobs overseas and denounced the hostilities that many young white-collar workers voiced toward old-line unions.

In a labor movement that had been declining since 1979, when union membership peaked at 21 million, Mr. Sweeney prodded his constituent unions to greatly increase spending on organizing. He often said that his first priority was to reverse the long slide and substantially expand labor’s rank-and-file.

But by 2009, when he stepped down, his vision of a dramatic unionization surge comparable to those of the late-Depression 1930s and the postwar ’40s had failed to materialize. In fact, overall union membership in America had fallen on his watch to about 12 percent from 15 percent of the workforce, a trend that has since continued, according to the United States Bureau of Labor Statistics.

“Based on the optimism that supporters of the labor movement felt in 1995 when he was elected, I think it’s hard not to be disappointed with the results,” Richard W. Hurd, a professor of labor relations at Cornell University, told The New York Times in 2009. “How much of that you can trace back to John Sweeney is a whole other question.”

In a departing interview with The Times in his Washington office — looking across Lafayette Park to the White House, where he had conferred with President Bill Clinton in the late 1990s and with Mr. Obama more recently — Mr. Sweeney spoke optimistically in the face of the Great Recession, which had been underway for more than a year and had already forced thousands of layoffs, further winnowing union ranks.

“I think the recession is going to drive people to the conclusion that they can’t resolve their problems by themselves, and they have to look to organizing,” he said. And, noting that his father had been a unionized New York City bus driver, he drew a lesson from childhood.

“Because of the union, my father got things like vacation days or a raise in wages,” he said. “But my mother, who worked as a domestic, had nobody. It taught me from a young age the difference between workers who are organized and workers who were by themselves.”

John Joseph Sweeney was born in the Bronx on May 5, 1934, to James and Agnes Sweeney, Irish-Catholic immigrants whose struggles in America had shaped John’s social perceptions from an early age. The boy had accompanied his father to many union meetings, where he learned of class and workplace inequalities and of union efforts to improve wages and working conditions.

He attended St. Barnabas Elementary School and graduated from Cardinal Hayes High School in the Bronx in 1952. Coming of age, he resolved to find a future in organized labor. He worked as a gravedigger and building porter (and joined his first union) to pay his way through Iona College, a Catholic school in New Rochelle, N.Y., where he earned a bachelor’s degree in economics in 1956.

He worked briefly as a clerk for IBM but took a sharp pay cut to become a researcher for the International Ladies Garment Workers Union in Manhattan. He met Thomas R. Donahue, a union rep for the Building Service Employees International Union, Local 32B, who persuaded him in 1960 to join his union as a contract director. Mr. Sweeney would face Mr. Donahue in a run for labor’s top job 35 years later.

In 1962, Mr. Sweeney married Maureen Power, a schoolteacher. She survives him, along with their children, John Jr. and Patricia Sweeney; two sisters, Cathy Hammill and Peggy King; and a granddaughter.

The building employees union was one of the most progressive of its day, representing 40,000 porters, doormen and maintenance workers in 5,000 commercial and residential buildings in New York City. Its contracts guaranteed pay raises, medical coverage, college scholarships for members’ children and requirements that employers hire and promote workers without regard to race, creed or color.

Mr. Sweeney rose through the ranks, and in 1976 was elected president of Local 32B of the renamed Service Employees International Union. Soon his 45,000 members struck thousands of buildings for 17 days and won major wage and benefit increases. He later merged Local 32B with Local 32J, representing janitors, and in 1979 struck again for contract improvements.

In 1980, he was elected president of the 625,000-member national S.E.I.U. and, moving his base to Washington, began merging with unions of public employees and workers in office jobs, health care and food services. He pushed for stronger federal laws for health and safety, and spent heavily to organize new members. By 1995, he represented 1.1 million union members and was a national power in the labor movement.

Labor was at a crossroads. Years of rank-and-file frustration with Lane Kirkland, president of the A.F.L.-C.I.O. since 1979, boiled over in a revolt of union presidents in 1995. Mr. Kirkland, whose internationalist vision of labor had made him a hero to Poland’s Solidarity movement but had left him unmoved, even hostile, to proposed reforms for unions at home, was forced to resign.

The 1995 election pitted Mr. Sweeney against Mr. Donahue, his old friend from Local 32B, who had risen to secretary-treasurer of the federation and was Mr. Kirkland’s heir apparent. But Mr. Donahue’s ties to Mr. Kirkland forced him to defend the status quo, and Mr. Sweeney’s progressive calls for growth and change won the presidency with 57 percent of the delegates, representing 7.2 million members.

He was re-elected to four more terms of two to four years each, the last time in 2005, when he broke a pledge not to remain in office beyond age 70. He retired in 2009, at 75, and was succeeded by Richard L. Trumka, his longtime secretary-treasurer and a former president of the United Mine Workers.

In a statement posted on the A.F.L.-C.I.O.’s website on Monday, Mr. Trumka said of Mr. Sweeney: “He was guided into unionism by his Catholic faith, and not a single day passed by when he didn’t put the needs of working people first. John viewed his leadership as a spiritual calling, a divine act of solidarity in a world plagued by distance and division.”

Mr. Sweeney wrote a memoir, “Looking Back, Moving Forward: My Life in the American Labor Movement” (2017), and was the co-author of two books: “America Needs a Raise: Fighting for Economic Security and Social Justice” (1996, with David Kusnet) and “Solutions for the New Workforce: Policies for a New Social Contract” (1989, with Karen Nussbaum).

In 2010, President Obama awarded him the Presidential Medal of Freedom, the nation’s highest civilian honor. “He revitalized the American labor movement,” Mr. Obama said at a White House ceremony, “emphasizing union organizing and social justice, and was a powerful advocate for America’s workers.”

Alex Traub contributed reporting.