Most of the White Vote in Mississippi was for Ford, but Southern Evangelical Carter attracted just enough of the White Vote, and all of the Black vote so Carter won that one. No f'n clue about Virginia though. Possible that since Virginia had a rather smaller black population than other Southern States, and the fact that Virginia's whites were solidly for Ford.
But what made the Mississippi white vote so different from the white vote in Alabama or Arkansas?
Well, Strom Thurmond actually made a commercial (
http://www.livingroomcandidate.org/commercials/1976) in support of Ford, Thurmond was pretty popular in Mississippi, but I think Carter Populism played pretty well with Alabama and Arkansas.