His spokeswoman Holly Baird confirmed the death. Additional details were not immediately available.

A liberal Democrat from what is now Detroit’s 13th Congressional District, Mr. Conyers was first elected in 1964, becoming one of five African Americans in the House. His overwhelmingly Democratic constituents reelected him 26 times over a period spanning 10 presidents, from Lyndon B. Johnson to Donald Trump.

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As the longest-serving member at the time of his resignation, Mr. Conyers earned the title “dean of the House of Representatives,” and this job security allowed him to promote liberal, sometimes controversial causes that won him a national following.

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He co-sponsored the Voting Rights Act of 1965, which prohibited discrimination at the ballot box. His fierce criticism of the Vietnam War led to clashes with Johnson and landed him on President Richard M. Nixon’s “enemies list” of political opponents.

After the terrorist attacks of Sept. 11, 2001, Mr. Conyers voted against the USA Patriot Act because he said it would roll back civil liberties. He later suggested that President George W. Bush should be impeached, saying he misled the country ahead of the 2003 U.S. invasion of Iraq.

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Mr. Conyers’s twilight years were marred by allegations of sexual harassment. According to legal documents published by the online publication BuzzFeed in November 2017, several of his female staff members claimed that he had approached them to request sex and that he had engaged in unwanted touching and other impropriety.

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One former staff member received a settlement of more than $27,000 from Mr. Conyers’s office after alleging in 2015 that he fired her for not accepting his sexual advances. The congressman denied wrongdoing. But after the House Ethics Committee opened an investigation and numerous representatives called for him to step down in November 2017, Mr. Conyers quit his post as the top Democrat on the House Judiciary Committee. The next month, he announced his resignation after 52 years in office.

“My legacy can’t be compromised or diminished in any way by what we are going through now,” Mr. Conyers declared defiantly. “This, too, shall pass.”

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Before the scandal, Mr. Con­yers had been an inspiration to African Americans from Detroit to the Deep South and had become, in effect, a member of Congress at large.

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“In many districts around the country, black voters did not feel represented by their leaders, so they would reach out to African American congressmen like Con­yers,” said Michael Fauntroy, who interned for Mr. Conyers in the early 1980s and is now an assistant professor of political science at Howard University.

Mr. Conyers, in turn, urged skeptical African Americans to get involved in politics. One of his early mottos was: “Register, vote, run for office. It’s power that counts.” To better harness that power and secure passage of legislation on poverty, racism, human rights, unfair tax policies and health care, Mr. Conyers and 12 other African American House members founded the Congressional Black Caucus in 1971.

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Mr. Conyers strongly backed the Rev. Jesse Jackson’s 1984 campaign for the Democratic presidential nomination and was an early supporter of candidate Barack Obama, who was then a Democratic senator from Illinois. Yet Mr. Conyers also could be caustic about fellow Democrats to demonstrate that he was not blindly loyal to anyone.

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In 1979, he described President Jimmy Carter as a “hopeless, demented, honest, well-intentioned nerd who will never get past his first administration.” Decades later, Mr. Conyers criticized Obama for making foreign policy too dependent on military muscle. His intention, Mr. Con­yers said of Obama, was “to make him a better president.”

He presented himself as an emeritus member of the Washington establishment and had participated in many high-profile political battles.

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Mr. Conyers was the only member of the House Judiciary Committee to take part in impeachment proceedings against Nixon in 1974 for the Watergate bugging scandal and coverup, and against President Bill Clinton in 1998 for lying about his affair with White House intern Monica S. Lewinsky.

Mr. Conyers considered Nixon a criminal and helped draft articles of impeachment against the president before he resigned. However, he called the effort to impeach Clinton a Republican coup d’etat and “the most tragic event in my career.” He voted “no” when the House voted to impeach Clinton. Eight years later, Mr. Conyers became the first African American to chair the Judiciary Committee.

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All along, Mr. Conyers was a master of the politics of symbolism. He hired civil rights activist Rosa Parks, who worked in his Detroit office for 20 years. He introduced numerous bills calling for reparations for the descendants of slaves, an issue that resonated among black people but did not gain traction in Congress. More successful was his 15-year struggle to recognize King, the civil rights defender.

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Four days after King was assassinated on April 4, 1968, and with the support of his widow, Coretta Scott King, Mr. Conyers proposed the first of many bills calling for a federal holiday in his honor.

The proposal met resistance from Republicans, notably Sen. Jesse Helms (N.C.), who accused King of Communist sympathies and complained that only one other holiday, Columbus Day, was named after a person.

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Mr. Conyers kept pushing, millions of people signed petitions and entertainer Stevie Wonder pitched in with the hit single “Happy Birthday.” President Ronald Reagan in 1983 signed legislation setting aside the third Monday in January as Martin Luther King Jr. Day; the day was chosen because it was near King’s Jan. 15 birthday.

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In a 2008 interview, Mr. Con­yers called it “far and away the thing I am most proud of.”

But as the Detroit Free Press, his hometown paper, once described it, “For every brilliant move, there’s a dud.” Mr. Conyers was famous for missing votes on the House floor. Critics claimed that his effectiveness was dulled by growing arrogance and a refusal to compromise. He was prone to gaffes, prompting Time magazine political columnist Joe Klein to call him “foolishly incendiary.”

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When Mr. Conyers ran for mayor of Detroit in 1989, challenging Democratic incumbent Coleman A. Young, he announced his bid by saying, “Move over, Big Daddy, I’m home.” Mr. Conyers finished third in the primary and then lost again in 1993.

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Mr. Conyers could be a demanding boss. In addition to the allegations that he sexually harassed staff members, the House Ethics Committee investigated him for pressuring staff members to babysit his children and to chauffeur him to private events in government vehicles.

After an investigation that lasted more than two years, the panel announced a deal in 2006 in which it dropped the inquiry in return for Mr. Conyers’s promise that he would not ask his staff members to do nonofficial work for him.

In 2009, Mr. Conyers’s wife, Monica Esters Conyers — a former campaign staff member 36 years his junior — was convicted of bribery while serving on the Detroit City Council and was sentenced to 37 months in prison.

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John James Conyers Jr. was born in Detroit on May 16, 1929. He served in the Army Corps of Engineers during the Korean War. With the help of the G.I. Bill for veterans, he graduated from Detroit’s Wayne State University in 1957 and its law school in 1958.

His interest in public affairs was partly because of his father’s position as an international representative for the United Auto Workers and, for a time, Mr. Conyers worked as a labor lawyer. He also was a legislative assistant to Rep. John D. Dingell (D-Mich.), one of the few House members to serve even longer than Mr. Conyers.

In the early 1960s, local Democratic Party elders considered Mr. Conyers too young to pursue federal office. Despite their opposition, Mr. Conyers ran in the 1964 Democratic primary for what was then Detroit’s 1st Congressional District and won by a mere 45 votes. He then scored a landslide victory in the general election.