Sen. John Ensign of Nevada, facing an ethics investigation over his affair with a former campaign aide, announced Thursday that he would resign his seat effective May 3.
Ensign said in a statement that he would send Vice President Joe Biden a letter Friday making the resignation official.
The 52-year-old Republican announced in March that he would not pursue re-election. He acknowledged in June 2009 that he had an extramarital affair with Cynthia Hampton, a former member of his campaign staff, and that he had helped her husband, Doug Hampton, a member of his congressional staff, obtain lobbying work with a Nevada company.
The ethics investigation focuses in part on some $96,000 Ensign's parents gave to the Hamptons, which Ensign's attorney has characterized as a gift.
The Justice Department and the Federal Election Commission investigated, then dropped the cases with little explanation. The Senate ethics panel, however, named a special counsel to look into the matter.
Through it all, Ensign insisted he would seek re-election until his announcement in March, when he reiterated that he had not violated any laws or ethics rules. He also said at the time the investigation did not influence his decision to retire from politics after 2012.
But in his resignation notice Thursday, Ensign said the appointment of the special counsel shook him because he had hoped the investigation would end with the Justice Department.
"While I stand behind my firm belief that I have not violated any law, rule, or standard of conduct of the Senate, and I have fought to prove this publicly, I will not continue to subject my family, my constituents, or the Senate to any further rounds of investigation, depositions, drawn out proceedings, or especially public hearings," the statement said. "For my family and me, this continued personal cost is simply too great."
With Ensign gone from the Senate, the Ethics Committee will have no jurisdiction in the matter.
Democratic Sen. Barbara Boxer, who chairs the committee, and GOP Sen. Johnny Isakson, the committee's vice chairman, said in a statement Thursday that Ensign "made the appropriate decision" in stepping down.
"The Senate Ethics Committee has worked diligently for 22 months on this matter and will complete its work in a timely fashion," the senators said.
Republican Gov. Brian Sandoval will now appoint someone to serve the remainder of Ensign's term, and choosing U.S. Rep. Dean Heller or another Republican would give the party the advantage of incumbency in a highly competitive seat that could decide which party controls the Senate after 2012.
Sandoval endorsed Heller minutes after his campaign announcement in March. Democratic U.S. Rep. Shelley Berkley is also running and is considered the strongest Democratic contender.
Former Gov. Bob List, now a member of the Republican National Committee, urged the governor to appoint Heller to the seat as Nevada Democrats quickly began their campaign to influence Sandoval's decision.
"Nevada needs a senator who is focused on creating jobs and protecting our middle class, not ending Medicare as we know it and giving more tax breaks to the rich, like Dean Heller is trying to do," Nevada Democratic Party spokesman Zach Hudson said.
Ensign was the third Senate Republican to decide not to seek re-election, following Assistant Senate Republican Leader Jon Kyl of Arizona and Kay Bailey Hutchison of Texas.
How the affair began
The Hamptons were hired by Ensign in 2006 — Doug Hampton as a top aide in the Senate office and Cindy Hampton to work on the Ensign for Senate and Battle Born PAC campaign committees.
Ensign and Cindy Hampton began their affair in December 2007, according to accounts provided by Ensign's office and Doug Hampton. Doug Hampton discovered the affair through a text message and confronted Ensign on Christmas Eve, according to Hampton.
In April 2008, Ensign's parents, Mike and Sharon Ensign, gave the Hampton family $96,000. Ensign characterized the money as given "out of concern for the well-being of longtime family friends during a difficult time."
The Hamptons left Ensign's staff that May, but Doug Hampton began consulting for a firm founded by Ensign's closest adviser and run by former Ensign aides.
The affair didn't end until August 2008, according to Ensign. That same month, Doug Hampton was hired as an executive of Las Vegas-based Allegiant Air, an airline run by Ensign political contributor Maurice Gallagher.
Ensign publicly acknowledged the affair on June 16, 2009, after Doug Hampton contacted Fox News about his story.
Doug Hampton was indicted this March 24 by a federal grand jury in Washington, D.C., on charges he violated federal criminal conflict of interest laws.
This article contains reporting from NBC News, The Associated Press and Reuters.