Incumbent S.C. Senator Jim DeMint is facing the winner of the Democratic party primary, Alvin Greene. Greene is such a terrible candidate that Dems are claiming he's a Republican plant. That seems unlikely, as presumably the Republicans wouldn't have planted a candidate this bad. Firstly, they didn't need to, as DeMint is expected to trounce whoever the Dems put up against him. Secondly, they wouldn't have expected this character to make it through the primary.
Republicans, when planting candidates in a Democratic primary, usually don't go for an unemployed, barely literate candidate who is facing felony obscenity charges.
Greene has given several completely bizarre interviews, including this one with Keith Olbermann: