Mr. Sorensen was proudest of a work written in haste, under crushing pressure. In October 1962, when he was 34 years old, he drafted a letter from Kennedy to the Soviet leader, Nikita Khrushchev, which helped end the Cuban missile crisis. After the Kennedy administration’s failed coup against Fidel Castro at the Bay of Pigs, the Soviets had sent nuclear weapons to Cuba. They were capable of striking most American cities, including New York and Washington.

“Time was short,” Mr. Sorensen remembered in an interview with The New York Times that was videotaped to accompany this obituary. “The hawks were rising. Kennedy could keep control of his own government, but one never knew whether the advocates of bombing and invasion might somehow gain the upper hand.”

Mr. Sorensen said, “I knew that any mistakes in my letter  anything that angered or soured Khrushchev  could result in the end of America, maybe the end of the world.”

The letter pressed for a peaceful solution. The Soviets withdrew the missiles. The world went on.

Theodore Chaikin Sorensen was born in Lincoln, Neb., on May 8, 1928  Harry S. Truman’s 44th birthday, as he was fond of noting. He described himself as a distinct minority: “a Danish Russian Jewish Unitarian.” He was the son of Christian A. Sorensen, a lawyer, and Annis Chaikin, a social worker, pacifist and feminist. His father, a Republican who had named him after Teddy Roosevelt, ran for public office for the first time that year; he served as Nebraska’s attorney general from 1929 to 1933.

Lincoln, the state capital, was named for the 16th president. Near the Statehouse stood a statue of Abraham Lincoln and a slab with the full text of the Gettysburg Address. As a child, Mr. Sorensen read it over and over. The Capitol itself held engraved quotations; one he remembered was “Eternal vigilance is the price of liberty.”

Mr. Sorensen earned undergraduate and law degrees at the University of Nebraska and, on July 1, 1951, at the age of 23, he left Lincoln to seek his fortune in Washington. He knew no one. He had no appointments, phone numbers or contacts. Except for a hitchhiking trip to Texas, he had never left the Midwest. He had never had a cup of coffee or written a check.

Eighteen months later, after short stints as a junior government lawyer, he was hired by John F. Kennedy, the new Democratic senator from Massachusetts. Kennedy was “young, good-looking, glamorous, rich, a war hero, a Harvard graduate,” Mr. Sorensen recalled. The new hire was none of those, save young. They quickly found that they shared political ideals and values.