Seth P. Tillman, a key aide to the late Sen. J. William Fulbright who wrote many of the lawmaker’s most noted speeches and helped shape his powerfully influential opposition to the Vietnam War, died Nov. 16 at the Washington Home hospice in the District.

He was 82 and had Parkinson’s disease, said his son Andrew Tillman.

As the longtime chairman of the Senate Foreign Relations Committee, Fulbright became known for his role in turning the United States against its involvement in Southeast Asia. But few know of the extent to which Dr. Tillman helped alter history by working in private on the stands Fulbright took in public.

Dr. Tillman was a Capitol Hill intern fresh from a doctoral program in foreign affairs when he went to work in 1961 for Fulbright’s committee. He quickly became a principal aide in the Arkansas Democrat’s Senate office as well as on the committee.

“There was nobody . . . more important to the formation of Bill Fulbright’s thought on foreign affairs” than Dr. Tillman, Fulbright biographer Randall Bennett Woods said in an interview.

Seth P. Tillman, a key aide to the late Sen. J. William Fulbright, died Nov. 16. He was 82. (Courtesy of Family )

Much of Dr. Tillman’s influence derived from his philosophical kinship with his boss. Woods described him as Fulbright’s “intellectual alter-ego.” Both were internationalists, he said, and both were “staunchly anti-Communist.”

On Vietnam, Woods said, the “elements of dissent were present in Fulbright’s mind.” But it was Dr. Tillman, he said, who “helped Fulbright . . . understand how the Cold War was going off the rails.”

Dr. Tillman assisted Fulbright during his committee’s high-profile hearings on the war. He was widely described as a chief collaborator — if not the ghostwriter — behind some of Fulbright’s most important addresses and publications in the 1960s.

They included his 1964 foreign policy speech “Old Myths and New Realities” and the lectures that became Fulbright’s 1966 book, “The Arrogance of Power.”

Dr. Tillman emphasized Fulbright’s contributions to their projects. “I don’t want to be a goodie-goodie aide,” he once told the New York Times, “but the truth is that Fulbright had a great deal to do with those things.”

Besides Vietnam, Woods said, Dr. Tillman worked with Fulbright on Latin American and Middle Eastern matters. Norvill Jones, a former committee staff colleague, credited Dr. Tillman with influencing the senator’s views on presidential war powers. Fulbright regarded the Senate Foreign Relations Committee as a check on executive power.

Dr. Tillman remained with the Senate subcommittee on the Middle East for several years after Fulbright’s defeat in the 1974 primary.

Dr. Tillman later was a fellow at the American Enterprise Institute and did consulting work on Capitol Hill. In 1982, he joined Georgetown University, where he was a research professor of diplomacy until his retirement in 2004.

His publications include “Anglo-American Relations at the Paris Peace Conference of 1919” (1961), “The United States in the Middle East” (1982) and “The Price of Empire” (1989), which he co-authored with Fulbright.

At the time of his death, Dr. Tillman was at work on a biography of the former senator, who died in 1995. Dr. Tillman’s son, Andrew, plans to complete the volume.

Seth Phillip Tillman was born Aug. 26, 1930, in Springfield, Mass. His father owned a grocery; his mother ran a clothing store.

Dr. Tillman’s 1950s Army service included a posting in Germany. He held a bachelor’s and a master’s degree from Syracuse University and a doctorate from Tufts University.

Dr. Tillman was a Bethesda resident. Survivors include his wife of 34 years, Baldwin Reid Tillman of Bethesda, and two sons, Andrew Tillman of Cambridge, England, and Peter Tillman of New York City.