Mr. Craig had his defenders, including Republican Senators Michael D. Crapo, also of Idaho, and Arlen Specter of Pennsylvania, both of whom said Mr. Craig was within his rights to remain and try to clear his name.

“Senator Craig is entitled to make his decision and I respect it,” Mr. Specter said. “Disorderly conduct is not moral turpitude, and it is no basis for leaving the Senate.”

Mr. Crapo, a friend and ally on state issues, said he strongly supported Mr. Craig. “I look forward to working with him in the Senate as he resolves his legal battles,” Mr. Crapo said.

If Mr. Craig sticks to his plan, he could be unseated before his term expires only if he was expelled by his fellow senators. Expulsion requires a two-thirds majority and has been used to remove just 15 senators since 1789, according to the Senate Web site. Fourteen of the 15 were expelled because they were sympathetic to the Confederacy during the Civil War.

After the news of Mr. Craig’s plea was first disclosed on Aug. 27, Senate Republicans leaned heavily on Mr. Craig to leave, concluding that his acknowledged offense disqualified him from remaining. Senator Mitch McConnell of Kentucky, the Republican leader, called it unforgivable and the party ousted Mr. Craig, a popular lawmaker at home before this episode, from his committee leadership slots.

Republicans were also eager to quickly dispose of a scandal in light of the acknowledgment by Senator David Vitter of Louisiana that he had been involved with an escort service under police investigation for prostitution, and the problems of Senator Ted Stevens of Alaska, the longest-serving Republican senator in history, who is under investigation in a federal corruption inquiry.

Mr. Craig announced on Sept. 1 that he intended to resign at the end of the month, touching off speculation about his successor. On Thursday, Gov. C. L. Otter of Idaho let it be known that he had settled on a replacement if Mr. Craig stepped aside, but did not say who that would be.