Bostic finished well behind Sanford in the first round of voting, and is hoping his strong ties to district's religious conservative activists and GOP unease with Sanford's past marital infidelities will help catapult him to a win in next Tuesday's runoff.

The Maryland congressman is not the only Republican backing Bostic over Sanford: Former Sen. Rick Santorum (R-Pa.) campaigned with Bostic in the district on Wednesday.

Reps. Jeff Duncan (R-S.C.) and Mick Mulvaney (R-S.C.) also endorsed a candidate against Sanford, though that candidate finished behind Bostic and didn't make the run-off.

The winner will face Democrat Elizabeth Colbert Busch, comedian Stephen Colbert's sister, in the GOP-leaning district a month later.