Indianapolis ? Indiana Rep. Mark Souder, an eight-term Republican who promoted abstinence education, said Tuesday he’ll resign from Congress after admitting an extramarital affair with a part-time staff member.

Souder won a bruising primary just two weeks ago, and the resignation effective Friday could hurt the GOP’s chances of holding onto the Republican-leaning district in November in a year that many expect will favor the party.

Souder, an evangelical Christian who has championed family values and traditional marriage, apologized for his actions but provided no details during an emotional news conference at his Fort Wayne office.

“I am so ashamed to have hurt the ones I love,” he said as he battled tears. “I am sorry to have let so many friends down, people who have worked so hard for me.”

The announcement stunned many in political circles. “Most people in this part of the state are as dumbfounded as they could be,” said Bob Schmuhl, a political analyst and University of Notre Dame professor.

Republican Gov. Mitch Daniels said he was surprised and disappointed by Souder’s announcement.

“He did a wrong thing but now he’s done the right thing,” Daniels said.

A spokeswoman from Daniels’ office said a special election will be held to fill the vacancy. The winner would complete the remainder of Souder’s term that ends in January.