I discussed here the gradually decreasing decline in relative vote swings by state:
The next step was to do this calculation by counties. For each presidential election year in the graph below, I computed the interquartile range (that is, the 75th percentile minus the 25th percentile) of the swings in vote proportions for the Republicans for the 3000 or so counties in the United States. I exclude third-party votes. For each year, I computed the interquartile range for all the counties in the U.S. and also just for the counties outside of the South.
I only seem to have the data at hand back to 1968 which is why this graph only goes back to 1972. As with the statewide swings, there has been a steady decline, in this case much more dramatic in the South. The decline is gradual but we’re clearly at a lower level of variation now than we were 30 years ago.
It would be interesting to analyse this by average age of the county to see if it explains some of the loss of variation. Most western countries have an aging population and as people get older they become more set in their ways.