It was the end of a stunning week in which Mr. Craig fell from senior senator to Republican Party outcast amid reports that he had solicited sex in an airport restroom.
“What is best for Idaho has always been the focus of my efforts, and it is no different today,” said Mr. Craig, standing with family members and a handful of prominent state Republicans, including Gov. C. L. Otter.
“To Idahoans I represent, to my staff, my Senate colleagues, but most importantly, to my wife and my family, I apologize for what I have caused,” he said. “I am deeply sorry.”
The senator said his resignation would be effective Sept. 30.
After serving a decade in the House and nearly 17 years in the Senate, Mr. Craig had suddenly faced overwhelming pressure from national Republican leaders determined to prevent the matter from becoming a burden in the autumn Congressional session and next year’s election.
“For any public official at this moment in time to be standing with Larry Craig is in itself a humbling experience,” he said.
Governor Otter, also a Republican, will name a replacement. Mr. Otter said Saturday that he had not decided whom to appoint.