Case was very popular among New Jersey voters, but, by the late 1970s, the state's Republicans began to shift their allegiances to more conservative candidates. With a very low turnout in the 1979 primary election, Jeffrey Bell, a Ronald Reagan supporter who had moved to New Jersey expressly to challenge the incumbent for his Senate seat, defeated Case. Throughout his tenure in the Senate, Case had always faced his most difficult challenges during the Republican primary, but had prevailed until this point. Following the unexpected and surprising loss, Clifford Case began the transition to a more private life with his family. With modesty and humility, he declared, "I think I can honestly say that it's miraculous that I was in politics for forty years and managed to stay that long."
Case's influence in New Jersey politics was substantial. As a moderate who advocated fiscal responsibility while supporting a strong federal role in social welfare programs, his name was frequently mentioned as a possible gubernatorial candidate. Case saw himself as better suited for his work in the Senate, not viewing his skills as appropriate for the executive branch. Case was often at odds with conservative New Jersey Republican power brokers. For example, he opposed the 1964 nomination of Barry Goldwater, tried to block Richard Nixon's 1968 nomination, and successfully worked to secure Gerald Ford as the Republican presidential nominee in 1976.
After leaving the Senate in January 1979, Case served at Rutgers University as a visiting lecturer on political affairs, and published articles on the Congress and on ethics in government. He joined an international law firm in Washington, served on the Helsinki Watch Committee, and chaired the Board of Directors of Freedom House. He also enjoyed spending more time with his extended family.
In 1981 Case was diagnosed with lung cancer. He had surgery in August but succumbed on March 5, 1982. Following the funeral service at Rutgers' Kirkpatrick Chapel, Clifford Case was buried in New Cemetery in Somerville. His wife Ruth lived until 2003, supporting the Senator's legacy through contributions to Rutgers University.