Sheldon Silver, one of the most powerful figures in New York state government for two decades before his conviction on corruption charges, has died in federal custody. He was 77.

Silver, who served as the speaker of the New York state assembly, died on Monday, the federal Bureau of Prisons said, adding that the official cause of death would be determined by the medical examiner. Silver’s supporters had said he was in failing health from multiple medical conditions.

The Manhattan Democrat, who told a judge he prayed he would not die in prison, was serving a more than six-year sentence for using his clout in state government to benefit real estate developers, who rewarded Silver by referring lucrative business to his law firm.

Silver’s conviction ended a nearly four-decade career in the assembly. He first won a seat representing Manhattan’s Lower East Side in 1976, and became assembly speaker in 1994, a powerful position that made him one of Albany’s “three men in a room” negotiating annual budgets and major legislation with the governor and state senate leader.

In all, Silver served as speaker during the tenure of five New York governors, from Mario Cuomo to Andrew Cuomo.

He became known as an inscrutable and stubborn negotiator, blocking proposals so often he was sometimes called “Dr No”. He helped scuttle the former New York City Mayor Mike Bloomberg’s plan to locate a football stadium on Manhattan’s West Side. And he took the brunt of the blame for the collapse in 2008 of Bloomberg’s congestion-pricing plan for Manhattan, which would have charged electronic tolls for driving through the borough’s most highly trafficked neighborhoods.

“He was a fighter for his constituents and his work to rebuild lower Manhattan after the terrible events of 9/11 will never be forgotten,“ said the current assembly speaker, Carl Heastie, in a statement. “For years he was the lone voice in the room pushing back against many regressive policies that would have harmed so many New Yorkers, and he presided over landmark laws to improve the lives of our most vulnerable residents.”

Silver was the youngest of four children of Russian immigrants. His father ran a wholesale hardware store. As an adult, he and his wife had four children and lived in a lower Manhattan apartment blocks from his first home.

An Orthodox Jew, Silver was known to observe Sabbath even during the marathon negotiation sessions that preceded annual budget deadlines and the end of legislative sessions.

Over time, he became a symbol of Albany’s much-maligned opaque style of governance and, ultimately, a target of federal prosecutors.

Prosecutors accused Silver of trading his influence for money. In one instance, they argued that Silver persuaded a physician to refer asbestos cancer patients to his law firm so it could seek multimillion-dollar settlements from personal injury lawsuits, a secret arrangement that allowed him to collect about $3m in referral fees. In return, prosecutors said he directed hundreds of thousands of dollars in state grants to a research center run by the doctor.

His original 2015 conviction was tossed out by an appeals court after a US supreme court ruling that narrowed the definition of a corrupt act. He was convicted again at a second trial in 2018 tailored slightly to conform to the high court ruling.

But an appeals court ultimately threw out the conviction related to the asbestos cancer patients. Prosecutors decided not to retry him on that charge. In the part of his conviction that stuck, a court found that he had supported legislation that benefited real estate developers who were referring tax business to a law firm that employed him.

Silver begged for mercy ahead of his sentencing in a letter to the judge.

“I pray I will not die in prison,” Silver wrote, saying he was “broken-hearted” that he damaged the trust people have in government.

The Associated Press contributed reporting