A longtime Republican congressman from Indiana said he's resigning over an affair with a staffer.

Rep. Mark Souder gave a tearful apology after eight terms on Capitol Hill.

"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, twisted for political gain."

Souder is an evangelical Christian and is known for promoting conservative family values and abstinence education.

He has been married for more than 35 years, and has three children and two grandchildren.

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

A special election will decide who finishes Souder's term which ends in January. Indiana Gov. Mitch Daniels said he was disappointed in Souder's announcement.

"He did a wrong thing but now he's done the right thing," Daniels said.

Republicans are concerned that Souder's departure could hurt the party's chances of holding onto that district in the northeastern part of the state.