Nevada Sen. John Ensign, facing an ethics investigation stemming from an affair with a campaign aide, said Monday he would not seek reelection because he wanted to spare his family from an "exceptionally ugly" campaign.
"At this point in my life, I have to put my family first," Ensign told reporters at a news conference in Las Vegas.
Just two months into the 112th Congress, Ensign is the eighth Senate incumbent to opt out of running for a new term, out of 33 seats up in 2012.
The announcement is welcome news to national Republicans who have admitted the two-term Republican would be vulnerable in the race. GOP Rep. Dean Heller, who won three statewide elections as secretary of State, is considered the party's best candidate to retain the seat.
In February, the Senate Ethics Committee named a special counsel to investigate claims that Ensign violated ethics rules and federal law in the aftermath of his affair with Cynthia Hampton, a campaign aide married to Douglas Hampton, another Ensign aide and close friend of the senator.
Ensign acknowledged the affair in 2009, after Douglas Hampton threatened to go public. The senator later acknowledged that his parents had paid Cynthia and Douglas Hampton $96,000 after Douglas Hampton left his job in the senator's office.
The Hamptons have suggested the payment was severance, but liberal critics have termed it an improper campaign contribution to Ensign by his parents. Ensign called the payment a gift.
Ensign said his decision not to seek reelection was not influenced by the ongoing Ethics Committee investigation.
"If I was concerned about that, I would resign," he said.
Two other investigations have come out in the senator's favor. But the scandal threatened to overshadow Ensign's campaign and fundraising potential; Republicans have suggested he would not even have survived a primary election.
The Democratic Senate Campaign Committee said in a statement that the seat was "ripe for a Democratic pickup," citing trends in voter registration and the presence of President Obama at the top of the 2012 ticket.
Ensign was first elected to the House in 1994, part of a wave of Republicans who retook the chamber for the first time in four decades. In 1998 he lost a bid for the U.S. Senate against incumbent Harry Reid, now the majority leader, by fewer than 500 votes. He won an open-seat race in 2000.
Ensign will become the third Republican to announce his retirement. Four Democrats and one independent who caucuses with the party have announced they will not seek reelection. Republicans need to win at least three seats -- pending the outcome of the presidential election, which would determine a tiebreaking vote -- to win back a majority in the upper chamber.