Attorney Curtis Bostic, who was a distant second to Sanford in the party preliminary March 19, may have high-profile backing but political observers told The Hill it may be too little too late.
"It's Sanford's to win -- he's a professional politician and knows how to run a race," said Katon Dawson, a former leader of the South Carolina Republican Party.
The winner of Tuesday's runoff will face Democrat Elizabeth Colbert Busch in the May 7 special election, called after Rep. Tim Scott, a Republican, was appointed to the Senate to replace Jim DeMint, who resigned to lead the Heritage Foundation.
Sanford, 52, was a two-term governor when in June 2009, after saying he was going hiking on the Appalachian Trail, publicly revealed he had engaged in an affair with Maria Belen Chapur, an Argentine woman. He was censured by the state General Assembly following an ethics investigation into allegations he misused state travel funds to conduct his affair but avoided impeachment.