The change indicates in part that Reagan and his message was very popular.

The reduced rate of change in the South from blue to red is an indication of the remaining "Southern Democrats" who had Republican values and beliefs, but were still pissed off about the Republican Party being on the side of the Union in the Civil War and the Reconstruction.

What is really interesting is when we then look at the 2016 map. It is almost entirely deep red, with the main exceptions being in population centers, primarily on the west and east coasts.

- This indicates that there is a bigger political difference between urban and rural voters. It turns out that the economic descendants of Thomas Jefferson's constituency are more likely to vote Republican. Whoda thought it?

- This sharper division indicates a broader ideological difference between Republicans and Democrats. In the 1976 and even the 1980 maps, we do not see such a sharp division because the two parties. Both in terms of platforms and in terms of beliefs and values of the base, the parties were not particularly different except on key issues. Since then, the philosophies of the two parties have diverged, and the differences are less about how or to what degree, and are now in terms of what the governmental aims should be.

I think the population density has a lot to do with this. People who have more elbow room (in the red swath of the center of the country) want less government involvement. People in denser areas (in the blue spots in population centers) have less elbow room, so they seem to want government to control the problem of other people.