Answer: Kennedy, 1960. Since then we've had presidents come from (most recent relevant experience):
VP: Johnson, Nixon, Ford, Bush I
Governors: Carter, Reagan, Clinton, Bush II
No Congresspersons.
The reason? Laws and sausage. Neither are pretty to watch being made. The compromise necessary in being a Senator will always alienate EVERYONE at some point. Anyone with a hint of ideological purism will find something on which to reject a sitting Congressperson. Governors are not on a national stage and get to be the bosses in their little demesnes, so their hands are usually a little cleaner than those who have to legislate by committee.
Is this bad for prospective candidates like Hillary Clinton? Or will knowledge of this history allow us to overcome prejudices? One could argue either way from a strictly pragmatic POV. However, such knowledge
should cool some of the anti-Hillary rhetoric going on.
It's just too easy to reject a candidate because one doesn't like so-and-so for his or her vote on such-and-such. It's also too simplisitic to buy into a saint-and-savior cult of a candidate who has no record on anything, like Clark (not that I dislike him).
So Senators always lose general elections. But what are the circumstances? How often do they win the nomination? And who do they run against in the primaries? Here's some raw data with no analysis. Just food for thought. (Computing percentages would be deceptive, as there is too little data, and circumstances are too transitory to state any real trends.)
Losing candidates:
Senators: Kerry, Dole, McGovern, Goldwater
Governors: Dukakis
VPs: Gore, Mondale (4-year break), Humphrey
Incumbents: Bush I, Carter, Ford
Incumbents winning:
(I am omitting ’64 and ’68.)
Nixon ’72, Reagan ’84, Clinton ’96, Bush II ‘04
Primaries: Losing candidates since 1972; aggregate raw data by party:
(1972 was the first election year after the demise of the old party boss system)
Dems:
Senators: 27
Govs: 8
House: 6
NG: 5
Mayors: 2
Military: 1
Repubs:
NG: 9
Senators: 6
House: 4
Govs: 3
VP: 1
edit: NG indicates candidate never held an elective office