This is an interesting look at the topic: "Is the South Too Republican for Republicans?"
http://www.tnr.com/blog/the-stump/101634/the-south-too-republican-republicans
If this year's GOP presidential candidates have all year been making such a conservative pitch in order to appeal to a party shaped by the South, why have they been having such a hard time connecting with voters in the most Southern states of all? ...
Well, here's one thing to think about. What if the South has become so monolithically Republican that actual conservative proposals and argument of the sort that Santorum and Romney have been offering don't actually resonate all that much? ...
Such isolation, political science suggests, leads to more extreme positions, on either end of the spectrum (see the poll Monday showing that GOP primary voters in the two states are evenly divided on whether Barack Obama is a Muslim and are only marginally in support of interracial marriage.) But it also could make voters less attuned to the sorts of arguments and sound-bites that Romney and Santorum have become used to offering to Republicans who are more used to having it out with people in their own communities.