In the Senate, Thomas Eagleton was active in matters dealing with foreign relations, intelligence, defense, education, health care, and the environment. He was instrumental to the Senate’s passage of the Clean Air Act and the Clean Water Act, and sponsored the amendment that halted the bombing in Cambodia and effectively ended American involvement in the Vietnam War. Eagleton was one of the authors of The Hatch-Eagleton Amendment, introduced in the Senate on January 26, 1983 with Sen. Orrin Hatch (R), which stated that “A right to abortion is not secured by this Constitution”. In 1987, Eagleton returned to Missouri as an attorney, political commentator, and professor at Washington University in St. Louis, where until his death he was professor of public affairs. Throughout his Washington University career, Eagleton taught courses in economics with former chairman of the Council of Economic Advisors Murray Weidenbaum and with history professor Henry Berger on the Vietnam War. Eagleton selected research assistants from among his students. On July 23, 1996, Thomas Eagleton delivered a warm introductory speech for George McGovern during a promotional tour for McGovern’s book, Terry: My Daughter’s Life-and-Death Struggle with Alcoholism, at The Library, Ltd., in St. Louis, Missouri. At that time, McGovern spoke favorably about Eagleton and reminisced about their short-lived presidential ticket in 1972. During the 2000s, Eagleton served on the Council of Elders for the George and Eleanor McGovern Center for Leadership and Public Service at Dakota Wesleyan University. In January 2002, he joined other Missouri Democrats to oppose the nomination of former governor and senator John Ashcroft for United States Attorney General. Eagleton was quoted in the official Judiciary Committee record: “John Danforth would have been my first choice. John Ashcroft would have been my last choice.”

In 2005 and 2006, Thomas Eagleton co-taught a seminar on the US presidency and the Constitution with Joel Goldstein at Saint Louis University School of Law. He was also a partner in the St. Louis law firm Thompson Coburn and was a chief negotiator for a coalition of local business interests that lured the Los Angeles Rams football team to St. Louis. Eagleton authored three books on politics. Eagleton also strongly supported Democratic Senate candidate Claire McCaskill in 2006; McCaskill won, defeating incumbent Jim Talent. Thomas Eagleton led a group, Catholics for Amendment 2, composed of prominent Catholics that challenged church leaders’ opposition to embryonic stem cell research and to a proposed state constitutional amendment that would have protected such research in Missouri. The group e-mailed a letter to fellow Catholics explaining reasons for supporting Amendment 2. The amendment ensures that any federally approved stem cell research and treatments would be available in Missouri. “[T]he letter from Catholics for Amendment 2 said the group felt a moral obligation to respond to what it called misinformation, scare tactics and distortions being spread by opponents of the initiative, including the church.” Thomas Eagleton died in St. Louis on March 4, 2007, of heart and respiratory complications. Eagleton donated his body to medical science at Washington University. He wrote a farewell letter to his family and friends months before he died, citing that his dying wishes were for people to “go forth in love and peace — be kind to dogs — and vote Democratic”.