In 1972 Hathaway was the successful Democratic nominee for the U.S. Senate seat held by Margaret Chase Smith. In 1973, Hathaway was one of the three senators who opposed the nomination of Gerald Ford to be Vice President. (The other two were Thomas Eagleton and Gaylord Nelson.) Hathaway served from 1973 to 1979. He was an unsuccessful candidate for reelection in 1978, losing to William Cohen.