What's the matter with everyone, part 1

I have had several posts observing that the identity of "red" and "blue" states has changed a lot over the years.  Of course, every state is unique, but I have been trying to reduce the changes to a small number of basic patterns.  An obvious one is that the South has moved towards the Republicans.  However, that raises the question of what counts as the South.  I estimated a model in which the Democratic share of the vote in each state relative to the Democratic share nationally for the presidential elections between 1916 and 2012 was the product of an election score (how Democratic the South was) and a state score (how "Southern" a state is).  The eleven Confederate states and six border states (Oklahoma, Missouri, Kentucky, W. Virginia, Maryland, and Delaware) were allowed to have their own individual scores--all other states were put in one group.

Here is a histogram of the state scores, which are pretty much as you would expect.  

Here is a plot of the election scores by year:

Of course, the basic trend is that the South has moved towards the Republicans over the century.  The more interesting thing is that the movement was pretty much complete by 1972.  The South moved back a bit towards the Democrats in 1976 and 1980 (when Jimmy Carter was running), but apart from that there's been little variation since 1972.  I suspect that Southern whites have moved a bit more towards the Republicans during that time, but that's been offset by increased turnout among blacks.