Charles Hunt at Legbranch.com:
Gerrymandering, or even the redistricting process generally, has no bearing on county-level vote.
Therefore, if we observe similar declines in the number of competitive counties as we did in competitive congressional districts, we know that gerrymandering is not the sole cause. Using the same calculations Cook Political uses for congressional districts and applying them to county-level presidential results from the last 25 years, Figure 2 shows the resulting trend.
Not only do we observe the same trend in counties as we did among congressional districts, but the disappearance of swing counties is even sharper than that of congressional districts. While the number of swing congressional districts have declined by 56% since 1996, swing counties declined by 73% over the same period.
If gerrymandering, or even redistricting generally, was the main culprit in polarizing our politics or making our elections less competitive, then we would not expect significant changes in geographic areas for which the borders don’t change. Instead, we see that more and more counties are voting more and more consistently for one party or the other. At all geographic levels, and not just congressional districts subject to redistricting, voters are sorting into more like-minded and more partisan communities. This phenomenon of sorting is clearly a serious cause of this partisanship, regardless of whatever effects gerrymandering is said to have.