LAS VEGAS — Republican Sen. John Ensign of Nevada, damaged politically and facing a Senate ethics investigation over an extramarital affair, said Monday he won't seek re-election next year.

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.

Ensign, 52, acknowledged in June 2009 that he had an extramarital affair with Cynthia Hampton, a former member of his campaign staff, and that he had helped her husband, Doug Hampton, a member of his congressional staff, obtain lobbying work with a Nevada company.

In recent months, Ensign had been adamant that he would seek re-election and that he did nothing to violate the law or Senate ethics rules.

Ensign insisted the ethics investigation didn't affect his decision to retire, and he again denied he broke the law or ethics rules.

"It had zero effect," the senator 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 to replace Ensign.

"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.

Dan Albregts, an attorney for the Hamptons, did not immediately respond to phone and e-mail messages seeking comment.

Ensign's announcement in Las Vegas in some ways mirrored the 2009 disclosure of his extramarital relationship. For that conference, he quickly called reporters to the Las Vegas federal courthouse where he has an office to announce he had engaged in the nine-month affair.

Once elected with 55 percent of the vote, Ensign's admission that he cheated on his wife seemingly marked the beginning of his political downfall.

In just two terms, he had gained a foothold in the GOP's Senate leadership and had been discussed as a potential presidential candidate.

"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's parents provided the Hamptons with $96,000 that the parents described as a gift. Ensign also helped find Doug Hampton a lobbying job.

The Justice Department and the Federal Election Commission investigated then dropped the cases with little explanation.

The Senate ethics panel, however, recently named a special counsel to look into the matter.

Ensign acknowledged repeatedly in recent months that he was prepared for a tough election campaign, in which he was expected to face Rep. Dean Heller and Nevada Lt. Gov. Brian Krolicki in the GOP primary.

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 House in 1994, the 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, the Senate GOP's campaign arm. A 2009 trip to Iowa sparked speculation about a future presidential bid.

Ensign is the eighth senator to announce he will not seek re-election. They include Democrats Daniel Akaka of Hawaii, Jeff Bingaman of New Mexico, Kent Conrad of North Dakota and Jim Webb of Virginia; Republicans Kay Bailey Hutchison of Texas and Jon Kyl of Arizona; and independent Joe Lieberman of Connecticut.

Freking reported from Washington.