I'm old enough to remember; your analysis is wrong. People like Robert Byrd, Scoop Jackson, and George Wallace all ran in the 1976 Democratic primary. If you go back and look at the actual candidate list, there were plenty of Democrats running who where not considered liberal at the time.
Carter was running in the wake of the Watergate scandal and Vietnam war as well in the middle of major recession. The feeling in the country was that Washington and professional politicians had screwed up and could not be trusted. The public was looking for someone from outside Washington that came across as a non-politician.
And that is exactly how Carter portrayed himself. He downplayed his political experience as governor, and focused on things like being a peanut farmer from a small town who played softball and was a born-again Christian. The ultimate Washington outsider.
In that regard, the 1976 election and the 2012 election have a lot in common, as the public seems to be looking for the same thing in a candidate.
Democratic candidates
Jimmy Carter, former governor of Georgia
Jerry Brown, governor of California
George Wallace, governor of Alabama
Morris Udall, U.S. representative from Arizona
Henry M. Jackson, U.S. senator from Washington
Frank Church, U.S. senator from Idaho
Robert Byrd, U.S. senator from West Virginia
Sargent Shriver, former U.S. ambassador to France from Maryland
Fred R. Harris, former U.S. senator from Oklahoma
Birch Bayh, U.S. senator from Indiana
Lloyd Bentsen, U.S. senator from Texas
Terry Sanford, former governor of North Carolina
Milton Shapp, governor of Pennsylvania
I was 16 at the time but already following the grand game of politics. As I remember it, The press (the big 3 was all we had back then) had already wrote off the more “Conservative” candidate's. I also thing ole Teddy was expected to get in the race at any time.
I disagree with you, but many things were done in “smokey rooms” in those days. Whats more I was only 16. I still believe Carter slipped in as a unknown candidate that did not have the negatives of the others. In any case, we all know the results.
I only bring it up as a example of how a unknown candidate can sometimes have a edge that the “established” candidates lack. Mr Cain might well benefit from this and I hope he does.