LAS VEGAS — Just two weeks after taking the first steps toward a 2012 presidential bid, conservative Republican Sen. John Ensign of Nevada is admitting to an extramarital affair last year with a campaign aide.

Ensign, a rising star in conservative circles and Nevada's most popular Republican, stepped down Wednesday as chairman of the Republican Policy Committee, the fourth-ranking spot in the leadership.

Ensign disclosed the affair at a hastily arranged news conference, shattering his prospects for heading his party's ticket three years from now.

"Last year I had an affair. I violated the vows of my marriage," Ensign told reporters, refusing to take any questions. "It is the worst thing I have ever done in my life. If there was ever anything in my life that I could take back, this would be it."

Ensign, 51, belongs to the men's Christian ministry Promise Keepers, and has championed causes pushed by the Republican's conservative religious base.

Earlier this month, he went to Iowa, home to the nation's first presidential precinct caucuses, to speak as part of a conservative lecture series designed to define the Republican Party after its shattering defeat in last year's elections. Aides said the visit was about staking out a leadership position within the Republican Party.

"This really doesn't help a Republican Party that has tried to run as a party of family values," said Chuck Muth, a self-described conservative-libertarian activist. "It absolutely makes the party look hugely hypocritical."

Ensign did not disclose what prompted his decision to declare his infidelity.

Ensign's spokesman, Tory Mazzola, said the affair took place between December 2007 and August 2008 with a campaign staffer who was married to an employee in Ensign's Senate office.

In 2002, Ensign missed several public appearances and dropped official business for about two weeks to deal with what his aides then described as a family matter. A person familiar with that episode, speaking on condition of anonymity because of the sensitivity of the matter, said Tuesday the absence followed an earlier affair.

Ensign's admission complicates Republican Party efforts to unseat Democratic Majority Leader Harry Reid, also from Nevada, in next year's midterm congressional elections.