Sen. John Ensign (R-Nev.) announced Thursday he will resign his seat effective May 3.
"It is with tremendous sadness that I officially hand over the Senate seat that I have held for eleven years," he said in a statement. "The turbulence of these last few years is greatly surpassed by the incredible privilege that I feel to have been entrusted to serve the people of Nevada. I can honestly say that being a United States senator has been the honor of my life."
Ensign is under a Senate Ethics Committee investigation over claims he violated ethics rules in the aftermath of an affair with Cynthia Hampton, the wife of former top aide Doug Hampton, whom he allegedly helped obtain a lucrative lobbying job.
He cited the investigation in his statement.
"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. For my family and me, this continued personal cost is simply too great," he said.
The committee ramped up its investigation earlier this year, including hiring an outside counsel. The investigation would be dropped, however, if Ensign were no longer a member of the Senate.
The resignation announcement started speculation the committee was closing in the senator.
"First started hearing rumblings of Ensign resignation earlier this week. No other reason (outside of health) than ethics was moving ahead," Nevada political reporter Jon Ralston, who first reported the news, tweeted Thursday.
Ensign noted he was "hopeful" that the dismissal of other federal investigations would end the ethics investigation.
"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. 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," he said.
Reached Thursday night by phone, Ensign's lawyer, Robert Walker, who is representing him in regards to the ethics probe, declined to comment.
Democrat Harry Reid, Nevada's senior senator, said that Ensign had served the state well.
"He was a strong advocate for Nevada, and worked for many years to improve our state," Reid said in a statement.
"I know this is a difficult time for the family and I wish them all well as they work through it," he added.
Ensign said earlier this year he would run for reelection, but he was under heavy pressure from the party to step aside over concerns the fallout from his admitted affair would hurt their chances to retain his seat.
He announced in March he would not run for a third term.
Rep. Dean Heller (R-Nev.) has already announced he's running for Ensign's seat, and it's possible Republican Gov. Brian Sandoval would appoint him as Ensign's replacement.
If he's appointed, it would give Heller an advantage over Rep. Shelley Berkley (D-Nev.), his likely general election opponent.
Berkley was heavily recruited by Democrats to run and is considered the strongest competition for Heller. She quickly won the backing of Reid and the Democratic Senatorial Campaign Committee (DSCC).
Guy Cecil, Executive Director of the DSCC, expressed confidence in Berkley.
"There will be a very clear choice for Nevadans between an uncompromising extremist like Dean Heller, who wants to end Medicare and cut loans for small businesses to give more tax breaks for the very rich, and Shelley Berkley, a true fighter for Nevada’s economy and middle class. Nevada will remain a top target for Senate Democrats," he said in a statement Thursday.
Berkley does have to get through a primary, however. Byron Georgiou, a millionaire capable of self-funding, is also in the race. He has banked around $1.1 million, which includes $500,000 of his own money.
The congresswoman has a little more than $1.5 million cash on hand, according to her latest Federal Election Commission filing, while Heller has around $1.4 million.
Democrats have high hopes of winning the seat. The party has to defend 23 seats in 2012 and, if President Obama wins reelection, Republicans only need a net gain of four seats to win control of the upper chamber.
According to Nevada law, Sandoval would then have to call a special election for Heller's House seat. Republican Sharron Angle, who lost her bid to replace Reid last year, is running for Heller's seat.
Nevada Lt. Gov. Brian Krolicki (R) is also likely to run for Heller’s House seat but hasn't made a formal announcement.
Earlier this month, Hampton, Ensign's former chief of staff, plead not guilty to charges that he circumvented the federal "revolving door" law. The Department of Justice investigated allegations that Ensign violated federal law by helping Hampton obtain a job, but there were no charges filed.
Citizens for Responsibility and Ethics in Washington (CREW), which has long called for Ensign's resignation, put out a statement praising the senator's decision but pointing to the politics likely behind it.
"The truth no one is likely to admit is that Sen. Ensign is being pushed out to give the Republican party a leg up in the 2012 election by allowing Governor Brian Sandoval to appoint Rep. Dean Heller (R-Nev.) to the seat so he can run with all the advantages of incumbency," CREW executive director Melanie Sloan said.
-- Jordy Yager contributed to this report.
-- This post was last updated at 9:36 p.m.