Craig said earlier he planned to resign Sept. 30, then left the door open to stay if he could successfully withdraw his plea.
Craig, serving his third term as senator, pleaded guilty in August to disorderly conduct following a June 11 sting operation in a men's room at the Minneapolis airport.
That he will stay in the Senate past Sunday was an unwelcome development for Senate Republican leaders who have made clear they wish Craig would step down and let Idaho's GOP governor, C.L. "Butch" Otter name a replacement.
Otter canceled an extended tour of the state's overcrowded prisons this week to interview some of the nearly 30 people _ including Lt. Gov. Jim Risch and Attorney General Lawrence Wasden _ who have publicly expressed interest in Craig's job.
"He wanted to be ready to act if we received a letter of resignation," said Jon Hanian, Otter's spokesman in Boise. "Obviously, we had not. ... Until he receives a letter of resignation, we have no vacancy, therefore, there is no replacement."
Senate Minority Leader Mitch McConnell, R-Ky., told reporters he had nothing to add to previous statements in which he said he thought Craig made the proper decision on Sept. 1, when he announced his intention to resign by month's end.
Sen. Mike Crapo, R-Idaho, Craig's strongest ally in the Senate, said that Craig, "like every citizen facing allegations, deserves to be able to fully defend himself."
Craig's lawyer, Billy Martin, said it is "near impossible, and it should be" for Craig to withdraw his plea. But he said his client's conduct _ shoe tapping and hand gestures under a men's room stall divider _ was not criminal.
The arresting officer described those gestures as signals recognized in the gay community as an invitation for sex. Craig has repeatedly said he is not homosexual.
Jim Weatherby, professor emeritus of political science at Boise State University, said Craig has been permanently weakened by the charges.
"How effective can he be, when his leadership wants him out?" Weatherby asked. "I suspect they would want to punish him further than the steps they've already taken."
Craig gave up his leadership posts on Senate committees after his arrest become public. Senate Republicans would have to vote to restore those posts, an action seen as highly unlikely.
