Washington (CNN) -- Republican Sen. John Ensign of Nevada, who is the subject of an ethics investigation in the Senate, has announced he will resign his seat on May 3.
Ensign had previously said he would not run for re-election in 2012 following revelations that he had an affair with a female aide who was the wife of another top aide, and that his parents subsequently gave money to the aides' family.
"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," Ensign said in a statement released Thursday evening. "For my family and me, this continued personal cost is simply too great."
Ensign has admitted to having an affair with Cindy Hampton, a campaign aide and the wife of Ensign's former chief of staff, Doug Hampton. Investigators are looking into efforts by Ensign to assist the Hampton family by providing a nearly $100,000 payment to them, arranging lobbying work for Doug Hampton, and possibly meeting with Doug Hampton on a lobbying matter in violation of Senate rules.
In February, the Senate ethics committee announced the appointment of a special counsel for the Ensign investigation. According to Ensign, the Justice Department has previously told him it would not bring criminal charges in the case.
"I am gratified that, after extended investigations, both the Department of Justice and the Federal Election Commission saw no grounds on which to charge me with improper conduct," said Ensign's statement Thursday. "I was hopeful that, with the closure of these investigations against me the wear and tear on my family and me would soon be over. This was not the case."
Ensign, 53, was first elected to the Senate in 2000. He previously was a veterinarian and general manager of the Gold Strike Hotel and Casino.
After Ensign announced last month he would step down in 2012, two House members from Nevada, Republican Rep. Dean Heller and Democratic Rep. Shelley Berkley, declared they would run to succeed him.
Nevada Gov. Brian Sandoval, a Republican, is expected to appoint a replacement to serve the rest of Ensign's term. There is speculation that Heller would get the nod to give him an incumbent advantage in 2012.
A total of three incumbent Senate Republicans scheduled to face voters next year -- Ensign, Jon Kyl of Arizona, and Kay Bailey Hutchison of Texas -- are not seeking re-election. Four incumbent Democrats -- Kent Conrad of North Dakota, Jim Webb of Virginia, Jeff Bingaman of New Mexico, and Daniel Akaka of Hawaii-- are not running in 2012.
Sen. Joe Lieberman of Connecticut, an independent who caucuses with the Democrats, has also announced his intention to step down after 2012.
The Democrats, who currently hold a 53-47 Senate majority, are defending 23 seats in the chamber in next year's elections. Republicans, in contrast, are defending only 10.