It's not like DeMint won in a landslide in the first place. He ran against a total nut in Inez Tennenbaum. If the Democrats field a credible challenger, he could be in trouble. Although, since his reelection is in an off year, he might be in better shape.
The sad truth is, there aren't any credible challengers for us. As you have noted, Jim Clyburn can be a real jerk. Congressman Spratt seems like a possibility but I'm not sure he has statewide appeal. He's been a big advocate of health care coverage for children, which may not fly in Sanford country. What about Jim Rex? I know nothing about him.
Bottom line, unless Fritz Hollings is coming out of retirement...this is a Republican seat. Funny thing -- Hollings would still be younger than about six Senators! LOL