Why did Carter do so well in the plains?

[Southern Senator North Carolina Yankee]

Exactly.

The Southern States that Carter won, weren't by the margins of solid south standards, but the registration and identification as "Democrats" remained from that era with only a slight decline from registration in metropolitan areas that leaned more Republican then the rural areas. Therefore, that meant that large numbers of Democrats were voting Republican in the South. A good example of this was the Jessecrats in NC. Helms killed it in the now urber Republican rural areas in central NC between Raleigh, Charlotte and the Triad. At the time, they were still Democrats. Carter seems to have taken back some of these counties, even intruding into ancestrally Republican counties on the TN border.

Also in the north, the type of working class voters who were voting Republican and providing the victory margins, were likely union Democrats as well, since their suburban bases alone and the ancestrally Republican rural areas were not enough to contribute to victory on their own. Ford won Michigan, so he probably got a good number of Democrats there as well.