Indiana Republican Rep. Mark Souder announced Tuesday he would resign from Congress, effective Friday, because he had an affair with a staffer.

The eight-term congressman apologized for his actions but provided no details.

"I am so ashamed to have hurt the ones I love," he said at a news conference in Fort Wayne. "I am sorry to have let so many friends down, people who have worked so hard for me."

Souder's decision could damage the Republicans' chances of holding onto the GOP-leaning district in northeastern Indiana. Souder won a bruising Republican primary on May 4 with 48 percent of the vote and was to face the Democratic candidate who four years ago gave him the toughest challenge since he was first elected in 1994.

Souder, 59, said he would not be a candidate in the fall election. It will be up to Republican Gov. Mitch Daniels to decide whether to call a special election to fill the vacancy or wait until the November ballot.

"I sinned against God, my wife and my family by having a mutual relationship with a part-time member of my staff," Souder said. "In the poisonous environment of Washington, D.C., any personal failing is seized upon and twisted for political gain. I am resigning rather than put my family through a painful drawn out process."

Souder has been married to his wife, Diane, since 1974, according to the biography on his office website. They have three adult children and two grandchildren.

Souder won the Republican primary over car dealer Bob Thomas, who spent much of his own money on television commercials portraying Souder as a career politician who wasn't a true fiscal conservative. Souder countered by emphasizing his A-plus marks from the National Rifle Association and 100-percent rating from the National Right to Life Committee.

Souder had been expected to face a stiff contest against Democrat Tom Hayhurst, a former Fort Wayne city councilman who got 46 percent of the vote against Souder in 2006. Nevertheless, the 3rd Congressional District has a strong Republican tilt; John McCain carried the district by more than 10 points in the 2008 presidential election.

One possible Republican replacement for Souder is state Sen. Marlin Stutzman of Howe, who finished second to former Sen. Dan Coats in this month's GOP primary for the U.S. Senate.

Hayhurst said in statement that his thoughts and prayers were with Souder and his family.

"I'm not running for Congress to run against anyone, but I'm running because I think I can help change Washington and that will not change not matter who is in the race," Hayhurst said.

Souder's resignation continues a significant turnover among Indiana's congressional delegation this year.

Democratic Sen. Evan Bayh announced in February he wouldn't seek re-election, saying he had tired of Congress. Democratic Rep. Brad Ellsworth is leaving his southern Indiana seat to run for Bayh's position.

Republican Rep. Steve Buyer said he January he wouldn't seek a 10th term in the House after his wife was diagnosed with an incurable autoimmune disease.