Via Paul Rosenberg comes an interesting chart from Charles Franklin plotting the Democratic vote share among whites against the share of the electorate that's black in any given state. The results are pretty arresting:
Broadly speaking, the more African-American voters in a given state, the more monolithically Republican that state is likely to be. It's not white voters, in other words, who are racist, but white voters in states with lots of black voters. You can come up with any number of hypotheses to explain the finding, but what's important about the graph is that the race of the Democrat was not a particularly powerful variable. As Tom Schaller has argued, the best way to explain the voting preferences of Southern whites is race, but the best way to predict their behavior is to understand that racist voting has become simple partisan voting. If they were once voting for Republicans because they were racist, they're now voting for Republicans because they are Republicans. That doesn't mean many of them are not also racist, but their preference for Republicans is not something you can lift by simply running a white guy, and it's not particularly magnified when Democrats run a black guy.
Broadly speaking, the more African-American voters in a given state, the more monolithically Republican that state is likely to be. It's not white voters, in other words, who are racist, but white voters in states with lots of black voters. You can come up with any number of hypotheses to explain the finding, but what's important about the graph is that the race of the Democrat was not a particularly powerful variable. As Tom Schaller has argued, the best way to explain the voting preferences of Southern whites is race, but the best way to predict their behavior is to understand that racist voting has become simple partisan voting. If they were once voting for Republicans because they were racist, they're now voting for Republicans because they are Republicans. That doesn't mean many of them are not also racist, but their preference for Republicans is not something you can lift by simply running a white guy, and it's not particularly magnified when Democrats run a black guy. 
