Former South Carolina Gov. Mark Sanford (R) faced accusations from a Republican opponent Thursday that his past marital infidelity made him a "compromised candidate" for Congress. 

The charges were leveled by former Charleston city councilman Curtis Bostic, who will face Sanford next week in a GOP primary run-off for the House seat vacated by Sen. Tim ScottTimothy (Tim) Eugene ScottPaul clashes with Booker, Harris over anti-lynching bill Rand Paul holding up quick passage of anti-lynching bill With capital, communities of color can lead our economic revival MORE (R-S.C.).

"We will lose this [seat] and lose it because of this issue of trust," Bostic said during a debate Thursday night.

Previously, Bostic had shied away from attacks on Sanford's 2009 scandal, when, as governor, he disappeared for days and was later discovered to have been in Argentina having an affair.

"Trust is a crucial issue. In fact, it has become a crucial issue in this race," Bostic said. He added that Sanford was a "compromised candidate" and that he risked losing a safe House seat to Democrats if he wins the Republican nomination.

"Democrats are excited at the possibility of taking this seat back," Bostic said.

Sanford, who divorced in 2010 and is now engaged to the woman with whom he had the affair, acknowledged that he had erred — but said it should not exclude him from winning the nomination.

"My faults are out — exposed — and all I can say, I have learned mightily from all of those mistakes," Sanford said.

Voters will decide on Tuesday which of the candidates will face off against Democratic candidate Elizabeth Colbert Busch, the sister of comedian Stephen Colbert. 

Sanford won 37 percent support in the first round of the GOP primary last week, while Bostic finished a distant second with 13 percent. 

While Sanford is the favorite in the Republican primary run-off, one recent poll showed the former governor neck-and-neck with Colbert Busch in a potential showdown in the heavily Republican district.

But Sanford said Thursday that Bostic too would struggle against the upstart Democrat.

"Both of us have vulnerabilities that [Colbert Busch] would try to exploit," he said.

A sign the race may be tightening: Sanford went on the attack against Bostic, ripping him for missing a number of votes while he was on the Charleston city council. Bostic responded he'd missed the votes because his wife had cancer.