That revelation prompted the National Republican Congressional Committee to pull its support from the campaign Wednesday, a day after The Associated Press obtained court documents detailing the accusations from Sanford's ex-wife, Jenny.
The group, which had conducted polling and provided additional resources to the campaign, was blindsided by the news and said it wouldn't provide more funding or pay for television advertising because officials worried Sanford would have difficulty making inroads with women voters. That blow effectively leaves Sanford on his own with three weeks to go before Election Day.
The latest Federal Election Commission reports still show that Sanford had $272,000 on hand to about $210,000 for Elizabeth Colbert Busch, his opponent in the race for a vacant seat in the state's 1st Congressional District.
"Mark Sanford has proven he knows what it takes to win elections. At this time, the NRCC will not be engaged in this special election," said Andrea Bozek, an NRCC spokeswoman. The decision was first reported by Politico.
Gibbs Knotts, chairman of the political science department at the College of Charleston, said it's not an insurmountable problem if Sanford can stick to his small-government talking points and appeal to the GOP base.
"I don't think this is fatal for Sanford. It's just a bad day for Sanford," Knotts said. "He needs to be out there talking about the size of government, the federal budget deficit and the themes he did very well talking about during the primary."
The NRCC made its announcement just hours after Sanford issued a statement explaining why he was at his ex-wife's home on Feb. 3. Jenny Sanford filed a complaint the next day, saying his visit on that night and several other occasions violated their divorce settlement.
"I did indeed watch the second half of the Super Bowl at the beach house with our 14-year-old son because as a father I didn't think he should watch it alone," Sanford said. "Given she was out of town I tried to reach her beforehand to tell her of the situation that had arisen, and met her at the back steps under the light of my cellphone when she returned and told her what had happened."
She did not immediately respond Wednesday to a request for comment on her ex-husband's explanation.
As for Colbert Busch, she has refused to exploit the latest image problem of the man who once was mentioned as a potential presidential contender before his affair with a woman in Argentina and ethics violations left a once-promising career in ruins.
After a visit to a Mount Pleasant diner, Colbert Busch - the sister of comedian Stephen Colbert - refused to discuss Sanford's personal life or past indiscretions.
"We're going to focus on the positive message of job creation for this district," she said, after being asked several times about Sanford by reporters.
So far, voters in the conservative district have seemed open to forgiving Sanford for his past misdeeds and re-electing him to the seat he held for three terms in the 1990s.
He and Curtis Bostic emerged as the top vote-getters in a primary of more than a dozen candidates. He then defeated Bostic in a runoff, boosted by strong name recognition and his message of fiscal responsibility.
Sanford famously left the state for five days in 2009, with staffers telling the media he was out hiking the Appalachian Trail. Instead, he was in Argentina, visiting the mistress to whom he is now engaged.
Sanford tearfully admitted the affair during a news conference after he returned to the U.S., and he and Jenny Sanford divorced shortly thereafter. Before leaving office, Sanford avoided impeachment but was censured by the Legislature over state travel expenses he used for the affair. He also paid what is still the largest ethics fine ever in South Carolina at $70,000.
"I think what he did was silly and stupid, but he was still a good governor," said voter Stephanie Shackelford of James Island. As for the trespassing complaint "I don't see why we should even know about it. That's between them and their lawyers."
Sanford must now appear before a family court judge on May 9, two days after the election. During the hearing, he'll have to show why he should not be held in contempt for violating the couple's divorce settlement, which stipulates that neither may enter the other's home without permission.
Jenny Sanford has said the timing of the court complaint had nothing to do with her husband's political campaign.
However, it has some voters wondering if it demonstrates a pattern of poor judgment.
"I think it speaks to his morality and his ethics, and that just by itself gives me pause," said Dana Frazeur of Mount Pleasant. "I know that people should get second chances and everything, but when you are a figure at that level, you are held to a different standard and you should be."
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Associated Press Writer Ken Thomas contributed to this report from Washington.
