Colorado has had two one-term Senators in the past 30 years (not counting Ken Salazar). Democrat Tim Wirth narrowly won in 1986 to succeed Gary Hart and did not run for reelection in 1992. He was succeeded by Democrat turned Republican Ben Nighthorse Campbell. Republican Hank Brown won handily in 1990 to succeed William Armstrong, and he did not run for reelection in 1996. He was succeeded by another Republican, Wayne Allard.
EDIT: I see that Storr beat me in this regard. Well, my post stands.
edit: I found an interesting one: When Hawaii became a state in 1959, both Senate seats held elections. But, the class 3 seat was running for only a 3 year term since all of those seats would be up in 1962. Oren E. Long won the class 3 race then decided to retire instead of running for reelection in 1962.
I'm ignoring those who were appointed to fill a term but didn't run for a full one, won an election to finish a non full term but didn't run for reelection, pre-17th Amendment terms, or died in office during a first term (except for Long and RFK since they were assassinated).
Dick Kempthorne decided to retire and run for Governor of Idaho after one term in the Senate in 1998.
Illinois: William H. Dieterich retired after one term from 1933-39.
Indiana: Raymond E. Willis retired after one term from 1941-47.
Iowa: Thomas Martin retired after one term from 1955-61.
Harold Hughes retired after one term from 1969-75.
Louisiana: (Can't believe I didn't think of this one earlier) Huey Long was assassinated during his first term in the Senate in 1935.
Maryland: Phillips Goldsborough retired to run for Governor in 1934.
Herbert O'Conor retired in 1952.
Massachusetts: Frederick Gillet retired after one term from 1925-31.
Marcus Coolidge retired after one term from 1931-37.
Paul Tsongas retired after one term (and resigned early to John Kerry, his already
elected successor seniority) from 1979-85.
New Hampshire: Henry Hollis retired after one term from 1913-19.
New Jersey: A. Harry Moore resigned during his first term to become Governor 1935-38.
Albert Hawkes retired after one term from 1943-49.
Robert Hendrickson retired after one term from 1949-55.
Robert Torricelli retired after one term from 1997-03.
Jon Corzine resigned during his first term which lasted from 2001-06.
New York: Robert F. Kennedy was assassinated during his first term which lasted from 1965-68.
I'll stop at my home state, North Carolina: (I'm not sure if it counts, but he had already chosen not to run for reelection and died of non-natural causes, so I'll add it)
John Porter East had already announced he was not running for reelection in 1986 after his first term in the Senate. Based on known evidence he committed suicide sometime between June 27 (when he was last seen, by his daughter) and June 29, 1986 (when he was found dead by an aide) by carbon monoxide inhalation from his automobile in his garage. There is still mystery as to why he decided to kill himself, but a plausible explanation is that it had something to do with being a paraplegic since 1955 after contracting polio while in the Marine Corps.
