There are apparently now two Republicans who have taken the formal step of forming an exploratory committee to run for president. We already had Herman Cain, and now we get Buddy Roemer. To refresh your memory: Roemer was governor of Louisiana for one term during the Bush administration. The George H.W. Bush administration. He was defeated for re-election in 1991, and then defeated again in a 1995 comeback.
Roemer joins a great tradition, most recently represented by Mike Gravel in 2008. I'll call them the Captain Bateson class of candidates -- they're oddly out of sync with normal time, just like Captain Morgan Bateson of the U.S.S. Bozeman in the classic Star Trek episode "Cause and Effect." Roemer will have been out of office just over 20 years; Gravel topped that, since he served in the Senate until 1981. It helps if they were defeated for re-election and then rarely heard from since.
2004: Carol Mosely Braun (defeated 1998)
1992: Paul Tsongas (did not run in 1984); Jerry Brown (termed out in 1983)
1988: Donald Rumsfeld apparently ran briefly then and in 1996, per wikipedia; gone from Defense after 1976
1984: George McGovern -- didn't leave the Senate until defeated in 1980, but still counts; Ruben Askew (did not run in 1978)
Who else? Does John Connelly 1980 count? Does Newt count this time around? He's been out of office forever, but very visible in the interim, unlike I think everyone on the list here.
Oh, and -- yes, we could call them Harold Stassen candidates, but I don't think that captures it as well.
Obviously, two of the ones I listed returned to their previous offices, which seems pretty weird to me, especially since it's so rare.