Haley, a legislator vying to become the state's first female chief executive and replace disgraced Gov. Mark Sanford, called the claim posted on the blogger's site Monday a smear. She questioned the timing two weeks before the primary, saying it was an attempt to derail her campaign, once considered a long-shot.
"I have been 100 percent faithful to my husband throughout our 13 years of marriage. This claim against me is categorically and totally false," Haley, a 38-year-old mother of two, said in a statement. "It is quite simply South Carolina politics at its worst."
The claim came from Will Folks, a conservative blogger who previously served as Sanford's spokesman. Folks said the relationship took place in 2007 when he did communications work for Haley, including writing speeches and news releases. He offered no proof of it and refused to go into any details.
"I'm not going to paint pictures," he told The Associated Press on the condition his statements would not be published before the allegation was posted on his blog.
Folks pleaded guilty to criminal domestic violence in 2005 and left the Sanford administration around that time. He is now a political consultant and runs FITSNews.com.
Political scientist Neal Thigpen said he thinks most political activists won't believe Folks, but some tea party supporters could be swayed to support another candidate.
"It seems to me the whole thing is designed to stop Rep. Haley's momentum," said Thigpen, a Francis Marion University professor. "If it's Nikki's word against Will Folks', I know who I'd believe. ... But something like this can do her great damage."
Haley, a three-term legislator, is in a tight race for the nomination with Congressman Gresham Barrett, Lt. Gov. Andre Bauer and state Attorney General Henry McMaster.
As word of the purported relationship spread, Haley went on at least two talk radio shows. Callers mostly backed Haley and liked her forceful denial. They talked about South Carolina's dirty politics and its reputation for what's been called a "good ol' boy" network. Internet comments were similar, though Folks' Web site had more support for him.
Folks said he owes apologies only to Haley and her family, and to his own wife for not revealing the relationship earlier in their marriage. He wed in 2008.
Folks left the Sanford administration about the time he received a 30-day suspended sentence for domestic violence. He kicked open the door at a home he shared with a lobbyist and shoved her into a piece of furniture, police said.