Rep. Mark Souder (R-Ind.) will resign for having an affair with a female member of his staff.
"It is with great regret I announce that I am resigning from the U.S. House of Representatives, as well as resigning as the Republican nominee for Congress in this fall's election," Souder said in a statement. The resignation will be effective Friday.
"I sinned against God, my wife and family by having a mutual relationship with a part-time member of my staff," he added.
Souder also criticized the "poisonous environment" of Washington.
"In the poisonous environment of Washington DC, any personal failing is seized upon, often twisted, for political gain. I am resigning rather than to put my family through that painful, drawn-out process."
The Indiana conservative, who was first elected in 1994, has staked out strong, socially conservative stances throughout his career, including tough stances against online gambling and illicit drugs.
According to a GOP leadership aide, Souder staff approached aides with Minority Leader John Boehner (R-Ohio) on Sunday. Boehner talked with Souder on Monday -- and on Tuesday morning, Souder announced his decision to resign.
Boehner spokesman Michael Steel told The Hill that the leader “has been perfectly clear that he will hold our Members to the highest ethical standards.”
The staffer with whom Souder had the affair, Fox News reported, was Tracy Jackson, a part-time communications aide in his district office. Jackson, according to Fox, is in her mid-40s and is married, and reportedly earner $12,541 last year for her work for Souder.
Souder is the second House lawmaker to resign this year because of alleged improprieties. Rep. Eric Massa (D-N.Y.) resigned earlier this year.
The four-term lawmaker won a split GOP primary in early May with under 50 percent of the vote. The Democratic candidate in the race is Ft. Wayne City Councilman Tom Hayhurst, who lost to Souder by eight percentage points in 2006.
Indiana Gov. Mitch Daniels (R) has the option to call a special election to fill Souder's seat, though there's nothing under state law compelling him to do so before this fall's general election.
While Souder won the primary, his withdrawal from the race means that his spot on the ballot will be filled by "a caucus of precinct committeemen whose precincts are within the congressional district," according to the Indiana secretary of state's office. Those committeemen have 30 days to fill Souder's spot after he's officially withdrawn.
A GOP nominee for a special election, if it is called, would be chosen in a similar way.
-- Molly K. Hooper contributed to this story.
Updated at 9:51 a.m., 10:46 a.m., and 12:02 p.m.