Las Vegas ? Sen. John Ensign of Nevada, a conservative Republican with evident presidential ambitions, abruptly announced Tuesday he carried on an extramarital affair for much of last year with a woman on his campaign staff, a confession that jarred his scandal-weary state as well as his party.

“It is the worst thing I have ever done in my life,” Ensign said in a brief appearance before reporters. He provided few details other than to say he does not intend to resign from Congress, and he did not disclose what prompted his decision to declare his infidelity. “If there was ever anything in my life that I could take back, this would be it,” he added.

Ensign, 51, belongs to the men’s group Promise Keepers, a Christian ministry. A member of the Senate Republican leadership, he was chairman of the party’s campaign committee leading up to the 2008 elections in which Republicans lost eight seats to Democrats.