Black sharecroppers in Georgia 1898.
Take a look at Strange Maps and notice how the cotton production map of 1860 mirrors almost exactly the blue Obama votes through the South.
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.