1968-->1980 swing map. Note that the four Deepest South states are confounded by the existence of Wallace in 1968, which is why they have enormous swings towards the winner, but most of the rest of the map doesn't have those issues. (Outside the Deep South, only Massachusetts and Utah have the deepest, >90% shade, indicating their margin swung more than 30% between 1968 and 1980).
I was kind of surprised by how many places there were where Carter '80 was still stronger than Humphrey -- even besides the Southern ones, this is also true of Vermont and two Mid-Atlantic states. (It's kind of bizarre just how Republican Maryland was in 1968/1972, looking back. Although also 1988 -- it might just be that it's underrated how much of a swing state it was in the second half of the Cold War because it disliked Reagan.)
