The Democrats’ most powerful county leader in New York City, Thomas Manton, is giving up his lucrative and lengthy law career in order to keep his unpaid job as head of the Queens County Democratic Organization.

County leaders throughout the state have exerted tight control over judgeships and the process of endorsing candidates for top offices. Mr. Manton, a former congressman, has played a key role in picking each of the last two speakers of the City Council.

Mr. Manton did not return several phone calls for comment.

Mr. Manton, 74, quietly retired from his partnership at Manton Sweeney Gallo Reich & Bolz LLP following the passage of a new rule that bars county chairmen and executive directors from working at law firms that handle lucrative work on the estates of people who die without wills or close relatives. The rule was created by the state’s chief judge, Judith Kaye, and took effect May 1 – although political leaders have a one-year grace period to comply.

“The courts remain the last major bastion of patronage for old-style ma chine-type organizations,” a public policy professor at Baruch College, Douglas Muzzio, said. “It’s very lucrative and it’s a source of patronage for the political leaders. Clearly the rule is designed to mitigate that.”

County political committees largely control surrogate courts, where unclaimed estate matters are handled, and the public administrators’ offices that assign those estates to lawyers. In Queens, the counsel for the administrator’s office is Gerard Sweeney, a partner in Mr. Manton’s former firm. His name is no longer part of the firm now called Sweeney, Gallo, Reich & Bolz LLP.

Mr. Muzzio said he is not surprised that Mr. Manton picked his political post over his law partnership.

“If it’s an either or, give me the county leader,” Mr. Muzzio said. “It”s a position of power, influence, and honor, etcetera, as well as money, ultimately.” The process of picking judges has come close under scrutiny in the last few years following allegations the county leader in Brooklyn, Clarence Norman, sold judgeships. Norman was never officially accused of selling judgeships, but related investigations led to his felony convictions for depositing campaign contributions into his personal bank accounts. Upon his first conviction, he was removed as a member of the Assembly, where he was the deputy speaker, and from his job as county leader.

County political organizations have played a dominant role in picking judges for decades, but their role was curtailed significantly in January by a federal court judge, John Gleeson, who said judicial conventions should be replaced in favor of direct party primaries.

The power Mr. Manton harnessed during his 20-year reign was demonstrated most publicly when he delivered a decisive bloc of votes in the City Council to elect speakers Gifford Miller in 2001 and Christine Quinn in 2005. As a result, the two most powerful committee assignments, land use and finance, have belonged to Queens council members.

Mr. Manton was elected to Congress in 1984, replacing Geraldine Ferraro. Two years later, the county’s Democratic chairman, Donald Manes, committed suicide after his role in a corruption scandal emerged. Mr. Manton became chairman nine months later said a state senator, John Sabini, who briefly served as county leader before Mr. Manton took over. Since that first election, Mr. Manton has not faced a challenge, according to another associate.

“He was seen as a figure who came up from a blue collar background,” Mr. Sabini said, referring to Mr. Manton’s work as a flight navigator in the Marines Corps, a salesman for IBM, and a police officer.

One of Mr. Manton’s law partners, Michael Reich, downplayed Mr. Manton’s decision to quit so soon after the new rule took effect forbidding county leaders from handling court-appointed trusteeships.

“He could have stayed for another year,” Mr. Reich told The New York Sun. “Tom Manton retired from the law firm to spend more time with his wife and family and to be a full-time county leader.”

Mr. Manton’s retirement from law will not end the close relationship between his former firm and the Queens Democratic organization. Mr. Reich, for example, will remain the party’s executive secretary – a position appointed by Mr. Manton.

While Mr. Reich said Mr. Manton’s career was planned for some time, it did surprise at least one person close to him. “His wife didn’t understand why he left his paying job for his non-paying job,” Mr. Reich said.