Robert E. Poli, the president of the nation’s air traffic controllers union whose striking members were fired and replaced by President Ronald Reagan in 1981 in a high-stakes showdown pitting organized labor against federal law, died Sept. 15 at his home in Meridian, Idaho. He was 78.

The cause was kidney and liver failure, said his son, Robert P. Poli.

The strike by PATCO (Professional Air Traffic Controllers Organization), Reagan’s subsequent breaking of the union and the hiring of replacement workers were among the most significant job actions of their time, said Joseph A. McCartin, a professor at Georgetown University and a specialist on labor and social history.

They “helped to define labor relations for the rest of the century and even into the 21st century,” he said, turning public sentiment away from striking as a legitimate labor tactic and further emboldening employers in the private sector to permanently replace striking workers.

Reagan’s hard line with the PATCO strikers six months into his presidency helped establish an image of him at home and overseas as a strong leader who would not be pushed around.

McCartin likened the PATCO strike in its historic significance to the Pullman Railroad strike of 1894 — which was put down by federal troops — and the Homestead Steel strike of 1892, which collapsed after the intervention of state militia and a union vote to resume work.

The PATCO work stoppage began Aug. 3, 1981, when at least 12,000 of the nation’s 17,000 air traffic controllers defied federal law and walked off their jobs, seeking higher pay, shorter hours, better equipment and improved working conditions in a long-simmering labor dispute.

There were widespread flight cancellations and delays, and 22 of the nation’s busiest airports were directed to reduce their scheduled flights by 50 percent.

That morning in the White House Rose Garden, Reagan declared, “I must tell those who failed to report for duty this morning they are in violation of the law, and if they don’t report for work within 48 hours, they have forfeited their jobs and will be terminated.”

A judge found Mr. Poli and the union in contempt of court for disobeying a back-to-work order, but the PATCO president stood his ground.

“This is total intimidation,” he said. “All it’s doing is making our people tougher. We are going to stay on strike as long as it takes.” At stake was “the survival of the workforce,” Mr. Poli declared, asserting that 89 percent of the air traffic controllers never made it to retirement because of the stresses and demands of their jobs.

Two days after the walkout began, Transportation Secretary Drew L. Lewis announced that at least 12,000 striking air traffic controllers had been terminated and would not be rehired “as long as the Reagan administration is in office.”

With non-striking controllers, supervisors, other staffers and transferred military controllers, the Federal Aviation Administration put together a patchwork quilt of personnel to direct airplane traffic until replacements could be trained.

On Jan. 6, 1982, Mr. Poli resigned as president of the union, declaring that with the beginning of the new year, “the time is right.”

The union had been decertified as the bargaining agent for the air traffic controllers. It faced $40 million in fines and had filed for bankruptcy. Picket lines remained at some sites across the country, but many members had found other work.

Robert Edmund Poli was born Feb. 27, 1936, in Pittsburgh. He served in the Air Force as a young man, then worked as an air traffic controller in Cleveland, where he became a local union leader.

He came to Washington in 1972 on his election as vice president of PATCO. He became president in 1980, when the union’s board of directors picked him over a sitting president, John Leyden, who was perceived as less militant.

Mr. Poli “looks more like a college professor than a union leader, his face framed by prematurely gray hair and a salt-and-pepper beard,” a 1981 Washington Post profile noted. “He is an imposing man, carrying 235 pounds on his 6-foot-2 frame. . . . He speaks softly but firmly, his words calm and rational but his tone carrying the conviction of a religious leader.”

He came to the presidency of PATCO after a period of guerrilla labor tactics — including wildcat strikes, sickouts and slowdowns — had, in the eyes of many members, proved ineffective. “We are going to act more like a union,” he said at the time.

Under Mr. Poli’s leadership, the PATCO executive board endorsed Reagan’s presidential candidacy in 1980. Reagan, in turn, promised to improve working conditions for air traffic controllers. As late as July 1981, it appeared that Mr. Poli and Lewis, the transportation secretary, had reached a tentative agreement that could have averted the strike. But after checking with officers of PATCO locals around the country, Mr. Poli changed his mind and urged union members to reject the tentative pact, and they did.

Mr. Poli’s marriages to Selma Leyden and Diane Gibbs ended in divorce. Survivors include his wife of 18 years, Marijess Zboray Poli of Boise, Idaho; two children from his first marriage, Laura Cholet of Frederick, Md., and Robert P. Poli of Gaithersburg, Md.; and four grandchildren.

For two years after the PATCO strike, Mr. Poli remained out of the public spotlight. He wrote a book, which was never published, his son said.

Later he managed real estate development around a golf course in Florida, worked as a labor negotiator for General Electric and ran a BMW dealership in Alexandria, Va.

In 2000 he retired and moved to Henderson, Nev., later relocating to Idaho.

He never regretted his strike decisions, his son said.