Looking at the red/blue county map, it’s pretty easy to correlate most of the blue counties with major urban population centers.
One thing that has me mystified, though, is the neat blue line bisecting otherwise red Alabama horizontally, seemingly following the path of Highway 80, as far as I can tell, and bleeding over slightly into Mississippi and Georgia on either side. Montgomery, Alabama’s capital and second largest city, is in the middle of the blue strip, but what about the rest of it? What’s the explanation of Alabama’s “blue belt”?
Update: Chris explains in comments that Alabama's "blue belt" is Alabama's black belt.