The cause was congestive heart failure, his son Jeffrey said.

Mr. Kampelman, who was a legislative counsel to Senator Hubert H. Humphrey of Minnesota in the 1950s, had become a prominent lawyer in Washington when he received a call from another Minnesotan, Vice President Walter F. Mondale, in 1980.

President Jimmy Carter wanted Mr. Kampelman to represent the United States at the coming East-West meetings in Madrid, in which the United States was seeking to bring the Soviet Union and some Eastern European countries into compliance with world human rights accords signed in Helsinki, Finland, in 1975.

Mr. Kampelman took the job reluctantly and only after being assured it would last just three months. Instead it lasted three years — deep into the first term of President Ronald Reagan, who retained Mr. Kampelman as lead negotiator. When an agreement was announced in July 1983, Mr. Kampelman was careful not to give the Soviets too much credit.

"We cannot in good conscience permit a limited negotiating success, important as we believe it to be, to make us forget, much to our regret, that signatures on a document do not necessarily produce compliance with its provisions," Mr. Kampelman said in Madrid, days after the agreement was announced.

His comments, recounting a laundry list of past Soviet violations, including the 1979 invasion in Afghanistan and the 1981 imposition of martial law in Poland, angered Soviet delegates.

The toughness and cleareyed patience he displayed in Madrid impressed the Republican administration. Less than two years later — even after Mr. Kampelman had served as an adviser to Mr. Mondale in his 1984 presidential campaign — President Reagan appointed him to lead arms-control talks with the Soviets in Geneva. The president said he had "no more important goal" in his second term than to reduce nuclear weapons.

Mr. Kampelman, who had no previous experience negotiating arms control, said of the Soviet Union in advance of meetings in March 1985: "We cannot wish it away. It is here and it is militarily powerful. We share the same globe. We must try to find a formula under which we can live together in dignity. We must engage in that pursuit of peace without illusion but with persistence, regardless of provocation."

Mr. Kampelman returned to his private law practice in 1989, but the negotiations eventually led to the 1991 Strategic Arms Reduction Treaty, reducing nuclear weapons. When Mikhail S. Gorbachev, the last president of the Soviet Union, toured the United States as a private citizen in May 1992, his schedule included a meeting with Mr. Kampelman.

Mr. Kampelman also served as chairman of WETA, the public television station in Washington, and was a founder and moderator of "Washington Week in Review" (now "Washington Week") on PBS.

Max M. Kampelman was born on Nov. 7, 1920, in New York City. (He had no middle name, only an initial.) His parents, Jewish immigrants from Romania, lived in the Bronx and sent him to Jewish schools. Mr. Kampelman graduated from New York University in 1940 and enrolled in law school in the university's night program soon afterward, working to support himself during the day.

Mr. Kampelman opposed Communism as a young man, but he also opposed war, and he requested conscientious objector status during World War II. He was eventually sent to the University of Minnesota, where he was one of 36 volunteers who served in an experiment to study the effects of severe weight loss through what essentially was controlled starvation.

Even as Mr. Kampelman's weight fell to 100 pounds, he continued to study toward his law degree at N.Y.U., which he received in 1945. He later received master's and doctoral degrees in political science from the University of Minnesota.

Mr. Kampelman began working as a Humphrey aide when he was mayor of Minneapolis in the 1940s and later joined his Senate staff. His political views underwent a transformation along the way: he went from being a declared pacifist to a centrist regarded as a hawk by left-leaning Democrats.

"The development of atomic and hydrogen bombs led me to doubt my earlier faith in the power of nonviolence to overcome evil in international relations," he said in an interview.

He left Mr. Humphrey's staff in 1955 — the two remained close friends, and Mr. Kampelman worked on Humphrey's failed presidential campaigns in 1968 and 1972 — and spent much of the next quarter-century in private law practice.

In addition to his son, he is survived by two daughters, Julia Stevenson and Sarah Kampelman, and five grandchildren. His wife, the former Marjorie Buetow, died in 2007; the couple were married for 58 years. A son, David, died in 2004, and a daughter, Anne Wiederkehr, died in 2006.