Past unsuccessful Presidential Candidates and their later (elective) political careers:
Al Gore: nothing
Bob Dole: nothing
Michael Dukakis: Served out remaining two years of his term as Governor of Massachusetts. Did not run for elective office thereafter.
Walter Mondale: Ran for Minnesota Senate, 2002. Lost.
George McGovern: Served out remaining two years of his Senate term. Won re-election, 1974. Lost re-election bid, 1980. Failed in his bid for nomination of the Democratic Party in the 1984 Presidential race.
Hubert Humphrey: Ran for Senate, 1970. Won. Won re-election to the Senate, 1976. Died in office.
Barry Goldwater: Ran for Senate, 1968. Won. Won re-election to the Senate, 1974, 1980, 1984. Retired, 1987.
Adlai Stevenson: No elective offices
Thomas Dewey: Re-elected as Governor of New York, 1946, 1950.
Wendell Wilkie: Never held elective office, before or after.
Alf Landon: Lived another 50 years, but did not hold another elective office (to my knowledge)
Al Smith: Nothing.
John W. Davis: Nothing
Bob LaFollette: Served out his Senate term until his death.
and so forth...
In fairness, the Senators seemed to do pretty well for themselves. Dole didn't do any more because he was too old, but McGovern, Humphrey, and Goldwater all did alright for themselves; Goldwater even lending his name to the Goldwater-Nichols Defense Reorganization Act of 1986.
