In 10th paragraph, corrects to say Boehner spoke by telephone with Souder -- rather than met privately with him

U.S. Republican congressman from Indiana Mark Souder (R-IN) (C), congressman Eliyah E. Cummings (L) and congressman John Shaddeg talk to the press after meeting the deposed king of Afghanistan Mohammad Zahir Shah at the king's residence in the outskirts of Rome in this file image from November 9, 2001. REUTERS/Paolo Cocco/Files

WASHINGTON (Reuters) - Republican Representative Mark Souder, a self-proclaimed evangelical Christian, said on Tuesday that he had an affair with a female staffer and would resign, effective on Friday.

“I’m resigning rather than put my family through a painful drawn out process,” Souder said at a hastily called news conference in his home district in Fort Wayne, Indiana.

Souder likely faced an investigation by the House of Representatives ethics committee if he did not step aside.

In a statement issued by his office, Souder, 59, said he had “sinned against God, my wife and my family by having a mutual relationship with a part-time member of my staff.”

David Wasserman, who tracks congressional races for The Cook Political Report, said the Republicans were likely to retain the seat because it serves “one of the most Republican districts in the state of Indiana.”

The Republican Party will first have to move quickly to come up with a candidate to replace Souder in November’s election.

Souder is the 20th House Republican to announce plans to resign or run for higher office in November. There are 19 House Democrats who plan to retire, run for higher officer or have lost their party’s primary.

Democrats now control the U.S. House, 254-177, but with polls showing plenty of voter anger at Washington, lawmakers are scrambling to retain the advantage.

Souder’s profile in “The Almanac of American Politics” quotes him as saying he is “most defined by the fact that I’m an evangelical Christian.” He also has been an advocate of abstinence in sexual education.

On the eve of his announcement on Tuesday, Souder spoke by phone with House Republican Leader John Boehner. Michael Steel, a Boehner spokesman, said he was not aware of the details of their conversation but “Boehner has been perfectly clear that he will hold our members to the highest ethical standards.”

Souder’s resignation comes a few months after a first-term House Democrat, Eric Massa, resigned amid allegations of sexual misconduct.

In 2008, Souder won re-election with 55 percent of the vote in his northeast Indiana district, which is home to many manufacturers.

Souder, who was an aide to former Senator Dan Coats, gained a reputation early in his House career for challenging Republican leaders, arguing that bills they were pushing were not conservative enough.

During his House career, Souder worked to toughen U.S. drug policy and was one of only 32 House Republicans who voted for the 2008 bailout of domestic automobile companies.

When he first ran for office, Souder promised to serve no more than 12 years. He is now in his 16th year in the House.