I looked at the Wikipedia articles on a couple of rural Southern counties. Both have had consistent, slightly shrinking populations since the early 20th century, both are currently about 2/3 black, 1/3 white. The number of voters has increased massively since the Jim Crow era. Roughly 10-20x. These counties have significant white populations, so this isn't simply explained by blacks voting. Interestingly, there wasn't one big jump in the 1968 election or whatever, but there has been a steady climb.

Anyway, a lot of libs think the white grievance politics of the right started as a reaction of whites to the Civil Rights movement. The reality is that the Civil Rights movement enabled it and may have given the vote to more whites than it did blacks.

(http://www.autoadmit.com/thread.php?thread_id=5767053&forum_id=2!#49218663)