Henry S. Ruth, who was a key figure in the federal investigation of the Watergate scandal in the 1970s, and who spent a year leading the special prosecutor’s office examining wrongdoing in the Nixon administration, died March 16 at an assisted-living facility in Tucson. He was 80. His wife, Deborah Mathieu, said he had a stroke.

Mr. Ruth, a lawyer who had served in the Justice Department and had investigated organized crime, joined the special prosecutor’s office as second in command to Archibald Cox soon after the office was created in May 1973.

When Cox was dismissed during the infamous “Saturday Night Massacre” five months later, Mr. Ruth kept the office running until Leon Jaworski took over as special prosecutor on Nov. 1, 1973.

Mr. Ruth stayed on as Jaworski’s chief deputy during a tumultuous period when dozens of Watergate prosecutions took place and as a constitutional crisis about criminal activity at the highest levels of government played out between Congress and the White House.

The scandal began in June 1972, when five men with ties to President Richard M. Nixon’s reelection campaign were arrested while trying to install eavesdropping devices in Democratic National Committee headquarters at the Watergate office complex in Washington.

Nixon was reelected in November 1972 and remained in office as Cox and the special prosecution unit began to explore the depth of corruption in the administration.

After refusing to turn over tape recordings of White House conversations to investigators, Nixon demanded that Cox be fired as special prosecutor. Attorney General Elliot Richardson and his deputy, William Ruckelshaus, resigned rather than dismiss Cox. It then fell to Solicitor General Robert H. Bork, as acting attorney general, to carry out Nixon’s order.

During the confusing events of that Saturday night — Oct. 20, 1973 — Mr. Ruth was met at the door of the special prosecutor’s office by FBI officers, who initially denied him entry. He was told that the office of special prosecutor had been abolished.

“Let me tell you something,” Mr. Ruth reportedly replied. “I’m going up there.”

As members of his staff began to gather, Mr. Ruth rallied their spirits and vowed to continue the special prosecutor’s mission.

“He had called the staff together and made a compact with them to remain in their offices and preserve the evidence they had,” Samuel Dash, counsel to the Senate Watergate committee, told The Washington Post in 1973. “But for Hank Ruth, there might not have been a Watergate staff at all when Mr. Jaworksi took over.”

Mr. Ruth later described the standoff between Cox and Nixon as “the most profound moment of Watergate.”

“It was pretty clear to us,” he said in a 1992 CBS News documentary, “that this act of trying to abolish our office, firing Mr. Cox, was just a straight obstruction of justice.”

In July 1974, the Supreme Court ruled 8 to 0 that Nixon was required to turn over his tapes.

“For the first time,” Mr. Ruth said, “you really had a ruling that a president of the United States is not above the law, [that] the law will prevail over a president’s desire to keep something secret.”

Nixon resigned Aug. 9, 1974, and was pardoned the next month by President Gerald R. Ford.

Under Jaworski, the special prosecutor’s office brought criminal indictments against many top officials. Former attorney general John N. Mitchell, former White House counsel John W. Dean III, and former Nixon aides H.R. Haldeman, John Ehrlichman and Charles Colson were among those who went to prison.

After Jaworski stepped down in October 1974, Mr. Ruth took his place as special prosecutor. He questioned Nixon about the actions of his subordinates and about his tapes — in particular, a missing segment of 181 / 2 minutes. But by the time he resigned as special prosecutor in October 1975, Mr. Ruth still wasn’t sure who had erased the White House tapes.

“In a lot of situations, people just don’t talk,” he told the New York Times. “It wasn’t as though we had a lot of cooperating witnesses in any of these matters walking into our office asking to be questioned.”

Henry Swartley Ruth Jr. was born April 16, 1931, in Philadelphia. He graduated from Yale University in 1952 and from the University of Pennsylvania Law School in 1955.

He was an Army intelligence officer and practiced law in Philadelphia before joining the Justice Department’s organized crime section in 1961 under Attorney General Robert F. Kennedy. In 1964, he was sent to Mississippi to enforce provisions of the newly passed Civil Rights Act.

In the late 1960s and early 1970s, he served on commissions examining organized crime, taught law at the University of Pennsylvania and held a top legal position in the administration of New York Mayor John V. Lindsay.

After Watergate, Mr. Ruth was general counsel of the United Mine Workers Health and Retirement Funds and a partner at the former Washington firm of Shea & Gardner, where he handled legal cases for President Jimmy Carter’s former chief of staff, Hamilton Jordan, and the president’s brother, Billy Carter.

Mr. Ruth later practiced in Philadelphia and, in 1987, testified against the Supreme Court nomination of Bork, who had fired Cox during the Saturday Night Massacre.

In the 1990s, Mr. Ruth wrote several columns for the Wall Street Journal critical of President Bill Clinton and what he called “presidential perjury and obstruction.”

Mr. Ruth moved to Tucson in 1988 and was associated with the Washington law firm of Crowell & Moring until 1994. In 2003, he published a book with law professor Kevin Reitz, “The Challenge of Crime,” examining trends in crime and law enforcement.

His marriage to Christine Polk ended in divorce.

Survivors include his wife of 21 years, Deborah Mathieu of Tucson; three daughters from his first marriage, Diana Ruth of Santa Fe, N.M., Tenley Ruth of Albuquerque and Laura Ruth of Montpelier, Vt.; and three grandsons.

Reflecting on the lessons of Watergate in 1992, Mr. Ruth said: “The sad residue of Watergate was so many people saw that their president had lied for 15 months and saw it so vividly and directly that a basic cynicism started in this country that has deepened, and deepened in a way where . . . I don’t believe anything that comes out of Washington.”