(CNN) - Call it the next hurdle in Mark Sanford's political redemption tour.
The former South Carolina governor, whose political career was sidetracked thanks to a well-publicized affair, faces a Republican primary run-off for his old congressional seat on Tuesday.
And Sanford's past infidelity is front and center in the battle for the state's first congressional district, which he represented from 1995 to 2001 before being elected the Palmetto State's governor for two terms.
"Trust is a crucial issue. In fact, it has become a crucial issue in this race," said Curtis Bostic, a former Charleston city council man who is the other GOP candidate in the runoff. Bostic claimed in last Thursday's debate between the two candidates that Sanford was a "compromised candidate" because of his extramarital affair with a woman from Argentina. The affair came to light in 2009, sinking any hopes Sanford had of making a bid for the GOP presidential nomination.
Sanford and his then-wife, Jenny, were divorced in 2010 and Sanford finished his second term as governor in January 2011. He's now engaged to the woman with whom he had the affair.
Sanford, who's touting his fiscal conservative credentials as he campaigns for congress, is also asking the public for a second chance, telling CNN that he was seeking "redemption" by running for office again. He came in first in a 16 candidate primary two weeks ago, but finished with 37% of the vote, far short of the 50% needed to avoid a runoff. But he was far ahead of Bostic, who narrowly edged out state Sen. Larry Grooms to finish second with 13% and will face Sanford in the runoff.
Sanford said his first-place finish was "incredibly humbling."
"The events of 2009 absolutely represent a failure on my part, for which there were and probably always will be consequences," added Sanford. "But that does not mean, because you have had failure in your personal life, that you cannot step back into life again."
But Bostic, who has won the support of some top social conservative leaders, disagrees, arguing that the congressional seat - which has been held by the GOP since 1981 - would become vulnerable if Sanford becomes the Republican nominee.
"A compromised candidate is not what we need. It's just not what we need. We need to secure this seat. It needs to be red," Bostic said, citing surveys that indicated that Democratic candidate Elizabeth Colbert Busch is leading Sanford.
But Bostic did not repeat his criticism of Sanford's infidelity at their second debate this past weekend.
Colbert Busch is the Democratic nominee in the race, and is the sister of Stephen Colbert, the satirist and host of "The Colbert Report" on Comedy Central. He's campaigned on his sister's behalf, recently telling Jake Tapper, CNN Chief Washington Correspondent and anchor of CNN's "The Lead," that "she's my sister, and I'm willing to, you know, break the jewel of my own creation to try to do something for her."
Colbert Busch, an official with Clemson University's wind turbine drive testing facility, is expected to have an uphill climb in the May 7 general election because the district is heavily Republican. GOP Rep. Tim Scott won re-election to the seat by 27 percentage points in last November's election. But when Republican Sen. Jim DeMint of South Carolina stepped down from his seat late last year to take over as the head of the conservative Heritage Foundation, GOP Gov. Nikki Haley named Scott to fill the seat, triggering the special election to fill his seat.