In the comments was a link to this post with maps comparing 2008 presidential voting to cotton production in the South.
From that link:
The bottom map dates from 1860 (i.e. the eve of the Civil War), and indicates where cotton was produced at that time, each dot representing 2,000 bales of the stuff. Cotton was King back then, and mainly so in the densely cultivated border area between Louisiana and Mississippi, and in an equally dense band of cotton cultivation starting west of the Mississippi-Alabama line, tapering out across Alabama, Georgia and South Carolina. Other cotton centres are the areas around Memphis and what appears to be Lawrenceburg in southern Tennessee.
The top map dates from 2008, and shows the results of the recent presidential election, on county level. Blue counties voted for Obama, red ones for McCain (darker hues representing larger majorities)... The pattern of pro-Obama counties in those southern states corresponds strikingly with the cotton-picking areas of the 1860s, especially along the Louisiana-Mississippi and Mississippi-Alabama borders (the pattern corresponds less strikingly and deviates significantly elsewhere).
The link between these two maps is not causal, but correlational, and the correlation is African-Americans. Once they were the slaves on whom the cotton economy had to rely for harvesting. Despite an outward migration towards the Northern cities, their settlement pattern now still closely corresponds to that of those days.
And when you put the two maps together...