(Click to enlarge.)
Here's a graph I just made, that shows the proportion of Democratic and Republican U.S. Senators, U.S. Representatives, and state Governors in the former confederacy states, over time.
The first data point is 1929-30 (prior to F. Roosevelt-to show that the high level of Democrats early on is not a function of the Great Depression), the second is 1953-55, and each successive one is the next session of Congress. Point 4 represents the years immediately following the Brown v. Board of Education decision that invalidated segregation in the South. Point 9 represents the years following the 1964 Civil Rights Act (which was bitterly resented in the South), and the last point represents the 1995-96 Congress, when Republicans took control of the House for the first time in over 40 years, and many Southern Democrats switched to the Republican Party.
Now I just need to figure out how to put the appropriate dates on the X-axis. Excel is a bit of a pain on that kind of thing.