35% for Carter in 1976 seems a little low, considering he actually won the state. Maybe I don't understand demographics entirely, but are there really enough minorities in Mississippi to let Carter win the state with only 35% of the "white" vote?
Yes. Mississippi's "Negro" population was reportedly 37 percent by the 1970 census. As Carter almost certainly won at least ninety percent of black voters in the state, less than 40 percent of whites was sizable enough for a victory.
Carter got 82% of the black vote nationwide, so he almost certainly did not win 90 percent in Mississippi.
Mississippi black voters generally vote more Democratic than the national average. For example, in 1992 and 1996, Clinton won 90 and 95 percent of black voters in the state respectively, while winning only 83 and 84 percent nationwide.
Also, a New York Times survey at the time suggested that the average percentage of black voters for Carter in the Deep South
was 95 percent.
