One of the most eccentric congressional races in recent memory tilted towards the Democrats on Monday night after the party's candidate, Elizabeth Colbert Busch, skewered former South Carolina governor Mark Sanford over his Latin American affair.

In the only debate of the election contest, Colbert Busch referred directly at one point to Sanford's weakest point: the lies he has told in the past. In 2009, while governor, his staff had told reporters he was hiking the Appalachian Trail when he was in fact visiting his mistress in Buenos Aires.

Opting for the blunt approach over subtlety, Colbert Busch spoke about his "leaving the country for personal purpose". It was the telling moment of the 75-minute debate at the Citadel in South Carolina and Sanford made it much worse when, asked to respond, he replied: "I could not hear what she said."

The two go to the polls in South Carolina next Tuesday to decide on the vacant House seat, with Sanford trailing in the polls. A gain for the Democrats will not have much impact on the House arithmetic, leaving the Republicans in control with a large majority. It would be no more than a morale-booster for the Democrats to win in a normally Republican district.

The significance of the race is purely personal. The question is whether Sanford, having been so humiliated by the affair in 2009, will be able to mount an extraordinary political comeback. His pitch in the debate, as throughout the campaign, was that he had learned humility. "You don't go through the experience I had back in 2009 without a great deal of humility," he said towards the end of the debate. Summing up, he said he had learned from the experience on "the way up and on the way down".

He sought to hold on to Republican core voters by portraying Colbert Busch – sister of the comedian Stephen Colbert, who has been helping fundraise for her in New York – as dependent on the former Democratic House speaker Nancy Pelosi, a deeply unpopular figure among Republicans, and the unions, almost as unpopular in South Carolina.

But Colbert Busch trumped this by reminding voters in the hall and watching on cable or by livestream of his duplicity over his affair and suggesting he had told untruths too on local issues, claiming to have supported the development of the port of Charleston and then reneging on his promise.

During exchanges on job investment she said: "What you are saying Mark is just not true." Politicians often make such accusations about one another but, given Sanford's history, the jibe carries more force.

Normally such a House race would not attract much national media attention but Sanford's eccentric behaviour has guaranteed it. At the start of the race he invited his divorced wife to work for him: she refused. It emerged in the middle of the race that two days after the election he will face a complaint in a family court of trespassing on his wife's property.

Colbert Busch was helped during the debate by one of the moderators who more than hinted at hypocrisy when he asked Sanford how he could reconcile having voted to impeach Clinton in the Monica Lewinsky affair. Sanford said, in a statement that was more about Sanford than Clinton: "Do you think President Clinton should be condemned for the rest of his life based on a mistake he made?"

Summing up, Colbert Busch opted for a well-rehearsed gimmick aimed at wooing fiscal conservatives. She promised, if elected, to take a 10% pay cut. But it was an unnecessary gesture: the debate was primarily about Sanford's personal life, not fiscal discipline.