Sen. John Ensign (R-Nev.) announced his retirement Monday, ending months of speculation over whether the scandal-plagued Republican would forge ahead with another campaign in 2012.

Citing the "pain" that a "very ugly campaign" would inflict on his family, Ensign announced at a news conference in Las Vegas that he would not seek reelection.

"I do not want to put my family, those that I care about, or this state through what would be a very ugly campaign that would ultimately cause a great deal more pain than has already been felt as a result of my actions," Ensign said, according to prepared remarks.

Calling it "the most difficult decision of my life," Ensign thanked his family, staff and others who have stood by him since the revelation of his affair with the wife of a former staffer. And, despite abysmal poll numbers, Ensign said his decision was not about his 2012 reelection prospects.

"As I have traveled across Nevada during this time, I have encountered many challenges, but also many offers of financial support and for volunteers on my reelection campaign," he said. "I cannot tell you how very appreciative I am to those who have stood by me during some of my darkest hours."

He does plan to serve out the rest of his term, according to Republican sources.

His retirement is another that favors the GOP in its drive to retake the Senate majority next year. Ensign was widely seen as the most vulnerable Republican incumbent up for reelection next year, and his exit means Republicans will likely get their candidate of choice next year — Rep. Dean HellerDean Arthur HellerMcConnell: Midterms will be 'very challenging' for GOP Singer Jason Mraz: Too much political 'combat' in Washington Trump gives GOP midterm pep talk MORE (R-Nev.).

With Ensign out of the race, Republicans expect Heller to jump in. The Nevada congressman was already moving toward challenging Ensign in the primary.

Democrats are touting the announcement as their chance to pick up the seat.

"Nevada is now an open seat, and ripe for a Democratic pickup. It remains high on our target list. Whoever Republicans field as their candidate will have a tough time holding onto this seat in a blue-trending state with President Obama at the top of the ticket. Democrats will have the resources needed to win this seat and just as important, will build a grassroots organization that matches 2008 and 2010," Guy Cecil, executive director of the Democratic Senatorial Campaign Committee, said in a statement.

And Republicans expressed confidence they would keep the seat.

"Next year’s Senate race in Nevada will now come down to a clear choice between two competing visions for our country – between a Republican candidate who believes in smaller government, fiscal responsibility and creating good, private sector jobs, and a Democrat candidate who believes in keeping our country on the same reckless fiscal path of more government and higher taxes.  Republicans welcome this choice and I am confident we will successfully retain this seat as we work to win back a new Senate Republican majority," National Republican Senatorial Committee Chairman John CornynJohn CornynOvernight Finance: House panel to take up bill toughening review of foreign deals | Trump acknowledges Cohen payment on disclosure form | Officials set for new round of China trade talks Groups urge Senate panel to reject Trump's pick for Louisiana-based appeals court House panel will consider bill to boost foreign investment review powers next week MORE (Texas) said in a statement.

As for Ensign, Cornyn merely said: "I thank Senator Ensign for his nearly two decades of public service."

Ensign had repeatedly said he intended to run for reelection next year and recentlytold The Ballot Boxthat he was gearing up his political and fundraising operation ahead of 2012. He has held a handful of Washington fundraisers this year.

But it's no secret that Republicans wanted him off the 2012 ballot. Heller is polling well ahead Ensign and leads every potential Democratic challenger for the seat in hypothetical general election matchups. 

Ensign admitted an affair with the wife of a former campaign aide in 2009, and he's under investigation by the Senate Ethics Committee for allegedly helping the aide, Doug Hampton, land work as a lobbyist. 

Ensign is the third Republican senator to announce his retirement this cycle. Sens. Jon Kyl (Ariz.) and Kay Bailey Hutchison (Texas) also won't seek reelection.

The watchdog group Citizens for Responsibility and Ethics in Washington said Ensign should resign. But it also called on the ethics panel to proceed "full-steam ahead" with its investigation.
“Senator Ensign’s decision not to run for re-election does not absolve the Ethics Committee of its responsibility to hold him accountable," Melanie Sloan, the group's executive director, said in a statement. "The senator has nearly two years remaining on his term and the investigation has been ongoing for over a year-and-a-half already. The Ethics Committee should not use Sen. Ensign’s announcement as an excuse to sweep the whole matter under the rug.  Senators need to know that actions have consequences."

This story was originally posted at 1:42 p.m. and last updated at 4:23 p.m.