LAS VEGAS (AP) -- GOP Senator John Ensign of Nevada said Monday he won't seek re-election.
Ensign, who is under investigation by the Senate Ethics Committee, said he decided last week against seeking a third term because he was worried about the effect of a campaign on his family.
"I'm putting them first, instead of my career," Ensign said.
He acknowledged in June 2009 that he had an extramarital affair with a former member of his campaign staff, and that he had helped her husband, a member of his congressional staff, obtain lobbying work with a Nevada company.
In recent months, he had been adamant that he would seek re-election and that he did nothing to violate the law or Senate ethics rules.
Ensign, 52, denied the ethics investigation weighed in his decision. He denied again that he broke the law or ethics rules.
"It had zero effect," he said. "If I was concerned about that, I would resign. That would make the most sense, because then it would go away."
Democratic Senatorial Campaign Committee chief Guy Cecil promised a grassroots effort to elect a Democrat.
"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," Cecil said.
Once elected comfortable with 55 percent of the vote, Ensign's admission that he had cheated on his wife seemingly marked the beginning of his political downfall.
His retirement announcement Monday in some ways mirrored his 2009 disclosure of his extramarital relationship, when he also called reporters to the Las Vegas courthouse where his Senate office is located for a last-minute press conference where he announced he had engaged in the nine-month affair.
"If there was ever anything that I could take back in my life, this would be it," Ensign said at the time. "I violated the vows of my marriage."
Amid the scandal, Ensign helped find Doug Hampton a lobbying gig.
The Justice Department and the Federal Election Commission investigated the alleged bribes then dropped the cases with little explanation. The Senate ethics committee, however, recently named a special counsel to look into the allegations.
The nonpartisan Cook Political Report had named Ensign the nation's most vulnerable incumbent and National Republican Senatorial Committee Chairman John Cornyn had repeatedly refused to vouch for Ensign's re-election campaign.
If Ensign survived the primary, he could have faced an equally brutal general election fight. A roster of popular Democrats including Rep. Shelley Berkley of Las Vegas, Secretary of State Ross Miller and Attorney General Catherine Cortez Masto were named by party leaders as prospective rivals.
Elected to the U.S. House in 1994, Ensign, a former veterinarian preached family values, Christian fellowship and fiscal responsibility. Ensign became a rising star after he was elected to the Senate in 2000, and was eventually named the head of the National Republican Senatorial Committee. A 2009 trip to Iowa sparked speculation about a future presidential bid.