The theory sounded plausible to me, and it has a lot of evidence behind it. I remember the southern strategy, the GOP talking points, the Lee Atwater quote that you can't say 'nigger, nigger, nigger' anymore. This post lists a lot more evidence, going era by era. However, the last data point is the Willie Horton ad of 1988. That's 26 years ago. Everything else in the post is interpolation:
"...they are the party where white racists are welcome, where 'Barack the Magic Negro' is funny, and people email each other photos of Obama with a bone through his nose..."I doubt that overt white racists are welcome in the GOP. Overtly racist statements aren't. As for tasteless jokes spread by email, all political parties have that, I'm guessing. I prefer individual accountability (name and shame) to across-the-board generalizations.
It's very difficult for me to believe that all GOPers or even most harbor racist attitudes. The idea of the inherent superiority of the white race has been so discredited in these days of Barack Obama, Colin Powell, and Neil deGrasse Tyson. Who still believes it's correct to keep blacks in their place? Nobody I know.
So the question has moved to covert racism. Those charges are easy to make, and impossible to prove or disprove. So, what are some of these charges based on? In this post, the author talks about the assumption among southern whites that all other southern whites detest Obama, so it's completely safe to "blithely say hateful and racist stuff about Barack Obama" to any white southerner.
Is this racism, or is it dislike of Obama for political reasons? There are reasons in abundance for conservatives to detest Obama: huge deficits, spending tilted toward poverty programs ("dependence"), the blocking of budget cuts, creation of an expensive new federal health program, repudiation of activist foreign policy. support of marriage equality, etc. Why assume it's racism? If a person is generally saying racist things, then the assumption makes sense. Otherwise, though, you can apply Occam's razor and assume the dislike is due to differences over political policies. This approach also avoids virulent anger from GOPers who are labeled as racist, and very much resent it. Who doesn't resent unfair labeling? Not just blacks and liberals.
Extras. Is the conservative distaste for poverty programs due to racism? Maybe yes for some. However, is all distaste for poverty programs due to racism? I hope not, because I'm ambivalent in supporting poverty programs. I worry a lot about dependence and the sapping of the work ethic.
As for whites in southern states becoming almost all Republican, it's partly due to their gradual migration from traditionally being Democrats. It's also due to voters aligning with parties that reflect their principles. Southerners tend to be more conservative (and more evangelical), so it only makes sense for them to migrate to the GOP.
Update 1/13/15. A House GOP leader, Steve Scalise, spoke in 2002 to a group aligned with David Duke, a Louisiana white supremacist. Does that show Scalise is a racist, or just a conservative hunting for votes? Try proving it one way or the other.