Representative John Conyers Jr., an advocate of liberal causes for five decades and the longest-serving African-American in the history of Congress, died on Sunday at his home in Detroit. He was 90.

His death was confirmed by a family spokeswoman, Holly Baird.

Mr. Conyers, a Democrat, resigned in 2017 after accusations of unwelcome sexual advances by two women. His lawyers denied the accusations, but both Paul Ryan, a Republican and then the speaker of the House, and Nancy Pelosi, the House Democratic leader at the time and the current speaker, found the complaints credible and demanded that he step down.

Mr. Conyers was the only member of the House Judiciary Committee to participate in impeachment inquiries against both Presidents Richard M. Nixon and Bill Clinton.

In 1974, he said impeachment of Mr. Nixon was necessary “to restore to our government the proper balance of constitutional power and to serve notice on all future presidents that such abuse of power will never again be tolerated.”

But in 1998 he argued that Mr. Clinton’s relations with the White House intern Monica Lewinsky did not merit impeachment. Republicans, he said, “say the president has to be impeached to uphold the rule of law, but we say the president can’t be impeached without denigrating the rule of law and devaluating the standard of impeachable offenses.”

“This is not Watergate,” he added. “It is an extramarital affair.”

In fact, on the day he stepped down, he again denied that he had harassed former employees and said he did not know where the allegations had come from.

But he died as one of many prominent men in politics, entertainment and journalism accused of sexual misconduct, often toward employees. Unlike many of the others, Mr. Conyers did not admit wrongdoing.