Most of Platt's voters voted Romney!
I strongly suspect that many of them are Republicans who feel a civic obligation to vote, but do not want to vote for either Sanford or a Democrat, so will be voting for Platt as a safe protest vote instead of staying home. There would be more of those safe protest votes if either the Libertarian or Constitution parties had put up a candidate.
Even if Platt's to the left of the Democrats? And besides, why do so many people think Nader cost Gore the 2000 election if some Republican-leaning voters who didn't necessarily want Bush didn't vote for him? I'm not arguing that they're right, I'm just asking you how that figures.
Anyway, I have no idea why a state with a large religious conservative population like South Carolina, in a party with a powerful religious conservative constituency, would nominate a womanizer like Sanford for Congress. The worst part about it is that he'll lose to, of all people,
Stephen Colbert's sister.