WASHINGTON — Senator Craig of Idaho lost a bid yesterday to withdraw his guilty plea in a men’s room sex sting but defiantly vowed to finish his Senate term, prolonging a headache for Republican leaders already facing a tough political climate.

Mr. Craig had announced plans to resign his seat by September 30 but wavered when he went to court in hopes of withdrawing his plea. He issued a statement yesterday on staying in the Senate shortly after Governor “Butch” Otter of Idaho, a Republican, relayed word that he had selected a replacement for Mr. Craig in the event of a vacancy.

“I have seen that it is possible for me to work here effectively,” Mr. Craig said in a written statement that disappointed fellow Republicans who have urged him to step down. Mr. Craig, 62, said he will not seek a fourth term in November 2008.

“He is ready to act should we receive a letter of resignation,” Jon Hanian, Mr. Otter’s spokesman, said. Mr. Craig said in his statement: “I will continue my effort to clear my name in the Senate Ethics Committee — something that is not possible if I am not serving in the Senate.”

The bipartisan ethics panel has already signaled it is reviewing details of Mr. Craig’s case, a step requested by Senate Republican leaders. His decision to stay and fight raises the strong possibility of public hearings — almost certain to be televised — centered on the issue of gay sex.

Senate Republicans made clear they wish Mr. Craig would leave office and let them forget the episode that has fueled jokes on late-night television for weeks. Idaho is likely to remain in the GOP column after next year’s election, but Mr. Craig’s insistence on finishing his term was received frostily by colleagues.

“Senator Craig gave us his word” that he would resign by September 30 if he could not overturn the guilty plea, said Senator Ensign, a Republican of Nevada, who chairs the GOP campaign committee overseeing next year’s Senate elections. “I wish he would stick to his word.

“It’s embarrassing for the Senate, it’s embarrassing for his party,” Mr. Ensign said. Asked if Mr. Craig staying would be a distraction for the party, Mr. Ensign said: “It may be a personal distraction for me.”

The Senate Republican leader, Mitch McConnell of Kentucky — said the “whole matter is before the Senate Ethics Committee and will be dealt with by Senator Craig and the ethics committee.”

Mr. Craig was arrested June 11 in a men’s room in the Minneapolis airport by an undercover officer. The officer said Mr. Craig had exhibited behavior consistent with seeking a sexual encounter.

Mr. Craig said he had panicked when arrested and pleaded guilty to disorderly conduct because an Idaho newspaper had been aggressively investigating allegations that he was gay. Mr. Craig says he is not gay.