COLUMBIA, South Carolina -- The governor of South Carolina, a rumored 2012 presidential contender, tearfully confessed to having an affair with a woman in Argentina, dealing the latest blow to a struggling Republican Party.

Gov. Mark Sanford, who caused a ruckus after he could not be located by staff, family and friends for several days, apologized to his wife and four sons and said he will resign as head of the Republican Governors Association.

"I've been unfaithful to my wife," he said in a bombshell news conference in which the 49-year-old governor ruminated aloud with remarkable frankness on God's law, moral absolutes and following one's heart. He said he spent the last five days "crying in Argentina." His family did not attend.

At least one state lawmaker called for his resignation. As a congressman, Sanford voted in favor of three of four articles of impeachment against Democratic President Bill Clinton citing the need for "moral legitimacy."

Sanford's infidelity admission adds to an ever-growing list of woes for the out-of-power Republican Party as it looks to rebound against President Barack Obama and his emboldened Democrats. The episode is another distraction for a party seeking a turnaround after disastrous losses in consecutive national elections as it confronts a stark reality: its less than diverse ranks are steadily shrinking, its regional heft is limited to the South and it lacks a leader of stature to guide the party back to power.

Excerpts of e-mail exchanges between the governor and his mistress were published online Wednesday by The State. The governor's office wouldn't discuss the e-mails with The Associated Press, but told The State it wouldn't dispute the authenticity of the messages.

One from the governor read: "I could digress and say that you have the ability to give magnificent gentle kisses, or that I love your tan lines or that I love the curve of your hips, the erotic beauty of you holding yourself (or two magnificent parts of yourself) in the faded glow of the night's light -- but hey, that would be going into sexual details."

The woman in question, who lives in Argentina, has been a "dear, dear friend" for about eight years but, Sanford told reporters, the relationship didn't become romantic until a little over a year ago. He's seen her three times since then, and his wife found out about it five months ago. He did not identify the woman.

"What I did was wrong. Period," he said. He refused to say whether he will leave office. 

Sanford did not answer directly whether the relationship with the woman was over. 

He did say, "I had, to the people of South Carolina, based on my boys, based on my wife, based on where I was in life, based on where she was in life, a place I couldn't go and she couldn't go."

Critics said he neglected his gubernatorial authority and put the state at risk by leaving the country without formally transferring power. It wasn't clear how his staff could reach him in an emergency.

Sanford revealed Wednesday morning that he had gone to Argentina for a seven-day trip. For two days after reporters starting asking questions, his office had said he had gone hiking on the Appalachian Trial, a well-known hiking route along 2,178 miles (3,505 kilometers) of the eastern U.S.

Sanford denied instructing his staff to cover up his affair, but acknowledged that he told them he thought he would be hiking on the Appalachian Trail and never corrected that impression after leaving for South America.

"I let them down by creating a fiction with regard to where I was going," Sanford said. "I said that was the original possibility. Again, this is my fault in ... shrouding this larger trip."

Meanwhile, first lady Jenny Sanford at first told The Associated Press she did not know where her husband went for the Father's Day weekend. She said in a statement Wednesday that she asked the governor to leave and stop speaking to her two weeks ago. The governor says he wants to reconcile, and his wife said he has earned a chance to resurrect their marriage.

"This trial separation was agreed to with the goal of ultimately strengthening our marriage," she said.

Jenny Sanford, a millionaire whose family fortune comes from the Skil Corp. power tool company, has been central to her husband's political career. She ran his congressional campaigns and his first race for governor. She was an almost daily fixture at senior staff meetings, and often could be seen driving a minivan away from the Statehouse in the mornings.

The two met when Sanford, who has an MBA, was trying his hand on Wall Street. She was working at a brokerage house when he entered a training program.

First elected governor in 2002, the former real estate developer has more than year remaining in his second term and is barred by state law from running again.

A former three-term congressman, Sanford most recently snared headlines for his unsuccessful fight to turn aside federal stimulus cash for his state's schools. His vocal battle against the Obama administration -- and libertarian, small-government leanings -- won praise from conservative pundits. Ultimately, a state court order required him to take the money.

The Republican Governors Association said Mississippi Gov. Haley Barbour would assume the duties as chairman.

Sanford's announcement came a day after another prominent Republican, Sen. John Ensign of Nevada, apologized to his Republican Senate colleagues after revealing last week that he had an affair with a campaign staffer and was resigning from the Republican leadership.