Bostic finished second in Tuesday’s Republican primary for the Charleston-area House seat vacated by Tim Scott (R) when he was appointed to the Senate last year.
Sanford’s campaign said Kuhn’s criticisms were a case of “sour grapes” because the former governor backed Kuhn’s opponent in a 2004 state Senate election.
The campaign also highlighted a letter Kuhn wrote in 2008 congratulating President Obama on his first presidential election victory.
“John Kuhn's history shows his credibility was gone a long time ago when it comes to espousing GOP principles," Joel Sawyer, Sanford’s communication director, said in a statement.
Despite Kuhn’s endorsement of Bostic, it’s unclear whether there’ll be a groundswell of resistance to Sanford by his former opponents.
That may hurt Bostic’s prospects of catching Sanford, who has a major cash and name identification advantage with less than two weeks until the runoff.
South Carolina state Rep. Andy Patrick (R), who finished fifth in the GOP primary, said he jumped into the race partly because he thought Sanford’s run was a bad idea.
But he declined to endorse either candidate in the April 2 runoff and said other GOP state lawmakers in the state have not begun to rally around Sanford or Bostic.
“I’ve been up in [the state capital] Columbia the last two days and don’t really get a sense that anyone is really coalescing around anyone,” Patrick said.
In fact, Sanford has locked up the endorsements of two first-round rivals — Charleston County School Board member Elizabeth Moffly and businessman Keith Blandford — though neither won more than 1 percent of the vote.
Patrick said Sanford faces a more difficult task convincing voters in the upcoming special election to rally behind him.
“I don’t know, but a lot of them seem to think that it was a bad idea for [Sanford] to run,” Patrick said. "That is one of the reasons I decided to run."
Teddy Turner, who won a little less than 8 percent of the vote, said Sanford would fare better than Bostic in the general election but declined to endorse either candidate “at this point.”
Turner said he is leaving for spring break with his family and would not be able to contribute, regardless.
Turner said it is difficult to switch from competitor to ally so soon after an election.
“You are fresh off a race where you are running against these guys,” he said. “It’s kind of really hard to kiss and make up.”
In a special election, Turner argued, Sanford’s high name recognition would outweigh the baggage that is associated with his name.
“It certainly hadn’t [hurt] so far,” he said. “Everyone’s got baggage … His is all out there so everybody knows Mark Sanford and knows his baggage. And it doesn’t seem to upset as many people as you would think it would.”
Kuhn disagreed, arguing that Sanford was vulnerable in the primary and would put the seat at risk for the GOP if he wins the runoff.
“Yes, absolutely he's vulnerable in the general. The vast majority of women in the Republican Party won't vote for him, and I'm worried they'll turn around and vote for [Democrat] Elizabeth Colbert Busch against him,” he said.
“Even someone like me who's a party loyalist won't back him, with his ethics violations and spending the state's money to visit his mistress.”
State Sen. Larry Grooms (R), who Bostic narrowly edged to make the runoff, also hasn’t endorsed anyone.
Repeated calls to Grooms went on answered on Thursday.
Rep. Jeff Duncan (R-S.C.) backed Grooms in the primary. But now that Grooms has conceded, Duncan’s campaign said it didn’t plan on backing another candidate in the runoff.