We pulled exit poll data for races from 1976 to 2014. Not every election is included here, but the trend is still obvious. Southern white voters have moved consistently to the right. (The data below shows the net margin of support for Democratic or Republican candidates from each racial group by region, relative to the overall margin of support in national House races. So a point on the left of the center line shows a demographic group that voted more Democratic than the nation on the whole.)
Update: The sample size for Western non-whites in 1980 was very small, so we've removed it. We've also adjusted the data for Northern groups for that year.
There's a lot of messiness at the top of this chart, but for the past two decades, it's been pretty consistent. Northeastern whites are the most Democratic, surprising no one. But the shift rightward by Southern whites -- predating the presidency of Barack Obama by a long time -- is significant. (Notice, too, that Southern non-whites are a bit more conservative than non-whites in any other part of the country.)
In fact, if you look only at the net change between 1976 and 2014 in how each group has gotten more or less Republican relative to the nation on the whole, the movement is stark.
This is not the only reason Mary Landrieu lost, but it's a big part of it. And it's a big part of why the South on the whole has gotten more Republican. For Democrats, the predicted demographic shift that has been prophesied for a while now can't come fast enough.
Some readers asked how the regions were differentiated. Here's a map showing how they're split.