(CNN) -- Sen. Larry Craig on Thursday said he was not resigning from the Senate despite a Minnesota judge denying his request to withdraw his guilty plea stemming from his arrest in a sex sting at an airport men's room.

"I am extremely disappointed with the ruling issued today," Craig said in a statement. "I am innocent of the charges against me. I continue to work with my legal team to explore my additional legal options."

"I will continue to serve Idaho in the United States Senate, and there are several reasons for that," the Republican said.

Craig's decision to not resign, despite promising last month to do so if the judge ruled against his motion, was not a surprise to fellow Republican senators, Senate GOP sources told CNN.

No one in the GOP leadership has talked to Craig about his decision, the sources said.

"There has been a sense around the Senate that he was going to stay," a Senate GOP leadership aide told CNN, saying there was "frustration and fatigue around the issue."

Craig, however, did get a show of support from fellow Idaho Republican Sen. Mike Crapo, who said in a statement that "Sen. Craig has the right to pursue his legal options as does any citizen, and I support his effort."

Hennepin County District Judge Charles Porter found that Craig had entered the guilty plea to to a misdemeanor disorderly conduct charge "accurately, voluntarily and intelligently," and that it was too late to withdraw his admission.

In a sharply worded 27-page order, the judge found that Craig had freely given his plea after extensive discussions with prosecutors and after waiving his right to an attorney.

"The defendant, a career politician with a college education, is of at least above-average intelligence," Porter wrote. "He knew what he was saying, reading and signing." Watch a report on the judge's ruling »

Craig's lawyer, Billy Martin, said that his client was weighing whether to appeal the judge's decision.

"We renew our arguments that it is manifestly unjust to deny Sen. Craig's request to withdraw his guilty plea. Sen. Craig continues his steadfast denial that any inappropriate behavior took place at the airport," Martin said in a statement.

Craig was arrested in June by Minneapolis-St. Paul International Airport police, who accused him of making sexual overtures in an airport men's room to an undercover police officer. He pleaded guilty to disorderly conduct in August, entering his plea by mail and paying a $500 fine. View a timeline of the Craig scandal »

Once the guilty plea became public, Craig argued that the plea was a "mistake" and denied having engaged in any inappropriate conduct. He said he pleaded guilty without legal advice out of fear that the allegations would be made public at a time when a Boise, Idaho, newspaper was investigating long-standing rumors about his sexuality.

"This pressure was entirely perceived by the defendant and was not a result of any action by the police, the prosecutor or the court," he wrote.

Martin said last week that no judge signed off on the plea -- and, more importantly, that what Craig did in the restroom did not constitute disorderly conduct.

But Porter dismissed that argument, saying the facts police presented "provide a sufficient, supplemental, factual basis for a conviction of disorderly conduct."

The judge also said the transcript of the dialogue between Craig and the arresting police officer did not show "an improperly aggressive interrogation."

"There was no manifest injustice in the pressures to plead as perceived by the defendant," Porter wrote.

Craig had said he would resign from the Senate if he could not get the guilty plea withdrawn by the end of September -- but then said he would await Porter's ruling before deciding whether to step down.

If Craig stays in the Senate, the chamber's Ethics Committee has indicated it will investigate his conduct.

Craig, a 62-year-old, three-term senator, is up for re-election in 2008, but advisers said he had decided against running for another term before the news of his arrest was made public.

The Idaho Republican, who is married and has aligned himself with conservative groups that oppose gay rights, has denied he is a homosexual.

Sen. Arlen Specter, R-Pennsylvania, who had advocated that Craig fight to withdraw his guilty plea, said he wasn't surprised by the judge's decision.

"It's a question of law," Specter said. "I thought he had a chance. No doubt it's difficult to withdraw a guilty plea."

If he does leave, Idaho Gov. Butch Otter has decided on a replacement for Craig -- but won't name his choice until Craig resigns, Otter spokesman Jon Hanian said.

Hanian said the governor has not received a resignation letter from Craig and has not talked to the senator since the ruling. He denied that Otter -- a longtime friend of the senator -- is pushing Craig to go, saying the governor "just wants to be ready."

The Republican Senate leadership canceled a scheduled news conference after the judge's decision was released. E-mail to a friend

CNN's Ted Barrett and Dana Bash contributed to this report

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