In 1976 I was 20 years old, in the USMC (Camp Lejeune, NC) and voted for Jimmy Carter by absentee ballot.
(Going from my recollection of events):
With the Watergate scandal and Richard Nixon, remember also tht Richard Nixon stepped down and was pardoned by VP Gerald Ford - people were pretty fed up with what was viewed then as corruption by the Republicans. The Democrats could have ran Mickey Mouse and won. So they did.
Carter was from the South, a Baptist who talked about being born-again, and was an officer on a nuclear sub. Looking at him as a candidate with the facts available to the voters at the time, he seemed to be a good choice to set the government right and keep America strong.
Just goes to show you, I guess...
With all the Democrats had going for them in 1976, they still almost blew it.