Mr. Sanford is regarded as a political lone wolf and has made numerous enemies even within his own party, which controls both houses of the state legislature. Scott H. Huffmon, a political science professor at Winthrop University in Rock Hill, S.C., said it was unclear whether there would be pressure on the governor to resign. “His opponents,” Dr. Huffmon said, “are sitting back trying to figure out if they’d be better off with a completely emasculated governor to deal with or if they’d be better off with André Bauer,” the lieutenant governor, who would take office if Mr. Sanford resigned.

Mr. Sanford made at least one state-sponsored trip to Argentina during the period of his relationship. In an interview late Wednesday, Daniel Scioli, the governor of Buenos Aires Province, said he met with Mr. Sanford on June 26, 2008, in La Plata, a town outside Buenos Aires. Mr. Scioli said the request for the meeting had come from Mr. Sanford’s office via the United States Embassy.

The governor was not known as a moralist but has frowned on infidelity and as a congressman voted to impeach President Bill Clinton after the Monica Lewinsky affair. “He lied under a different oath, and that’s the oath to his wife,” Mr. Sanford said at the time on CNN. “So it’s got to be taken very, very seriously.”

Mr. Sanford and his wife had joined an intensive Bible study group for couples in the last few months, according to William H. Jones, president of Columbia International University, a conservative evangelical college. Dr. Jones said the governor and his wife had been encouraged to join the Bible study by a longtime friend, Cubby Culbertson, a businessman in Columbia who teaches the Bible and who was thanked by the governor in his news conference.

At the news conference, Mr. Sanford said his friendship with the unnamed woman began eight years ago and became a romance about a year ago. He said he had seen her three times since then. The relationship was “discovered” five months ago, he said, and he had been trying to reconcile with his wife.

Mr. Sanford strongly implied that he had ended the affair. “The one thing that you really find is that you absolutely want resolution,” he said. “And so oddly enough, I spent the last five days of my life crying in Argentina.”