The politics of being caught in an airport sex sting here are tough enough for Idaho Sen. Larry Craig, but a legal fight against his guilty plea won't be any easier. The Republican on Wednesday relayed word that he would resign his seat by Sept. 30 only if he fails to withdraw last month's guilty plea to a charge of disorderly conduct. Legal experts say that it's tough to convince a judge to allow a guilty plea to be withdrawn, and that even if the tactic succeeds, it could backfire if it leads to a trial where the lurid allegations against the senator get a full public airing.

Craig has maintained he is innocent following his arrest by an undercover police officer who said the senator had behaved in an airport men's room like a man soliciting sex. He has said he mistakenly pleaded guilty to simply make the case go away.

He has already been sentenced. He paid $575 in fines and fees, and has a 10-day suspended jail sentence hanging over his head during his one-year unsupervised probation if he commits the same offense again.

He also signed each page of a three-page guilty plea agreement. On page one, just above his signature, are the words "I now make no claim that I am innocent of the charge to which I am entering a plea of guilty."

Craig had yet to file court papers on Wednesday seeking to challenge the plea in Minnesota.

"I think the odds are long," said Doug Kelley, a Minneapolis defense lawyer and former assistant U.S. attorney who was chief of staff to former U.S. Sen. David Durenberger. "I think he's got to show either that he was coerced into the plea, or that he didn't understand his legal rights at the time that he signed the document acknowledging his guilt."

Hoping to keep the matter quiet, Craig - a rancher before he was first elected to Congress nearly three decades ago - did not hire an attorney when he pleaded guilty. He's hired attorneys now, but they weren't commenting on Wednesday about their legal strategy.

A defendant who wants to overturn a guilty plea has to demonstrate a "manifest injustice" under the state's Rules of Criminal Procedure, said Steve Simon, a law professor at the University of Minnesota who has run a clinic for defense lawyers for 30 years. That's a high hurdle, he said.

"Very few motions to withdraw pleas are brought," Simon said. "Of those that are brought, few are granted."

He added, however, that in Craig's case "there are some very serious problems with the validity of that plea" because of the possibility that Craig may not have specifically waived his right to an attorney.

The police officer's Miranda warning to Craig covers only the police interrogation - not the later court process. In his plea agreement Craig waived five specific rights, including the right to a trial, but not his right to an attorney. Hennepin County's standard plea petition includes a waiver of a right to an attorney but that wasn't used in Craig's case, Simon said.

A July 20 letter from prosecutor Christopher Renz to Craig laying out the proposed plea agreement made several references to an attorney. "Please review the document and to the extent that you wish, review the same with legal counsel," the letter said. The Metropolitan Airports Commission, which prosecuted the case, declined to make Renz available for comment on Wednesday.

Minnesota's judicial rules require a defendant who doesn't use an attorney to waive that right, and such an omission has been the basis for withdrawing other guilty pleas, Simon said. He added that because Craig did not use an attorney, the prosecutor has the burden of showing the plea was valid. If Craig had had a lawyer, the burden would have been on the senator to show the plea was invalid.

To withdraw his guilty plea, Craig would have to bring the motion before the same judge who handled the case. If the judge agreed, the prosecutor would be free to refile the case - including a gross misdemeanor charge of interference with privacy that was dismissed in the plea agreement. That charge was based on a police sergeant's claim that Craig gazed into his stall.

Craig could then take the case to trial in search of an acquittal.

"Legally, I'll give him decent odds on that," Kelley said. "But PR-wise, he'll get murdered."

For starters, the prosecutor could be expected to put the police officer on the stand to testify about the protocol for bathroom sex. "I wouldn't think that would be much fun," Kelley said.

Whether or not Craig can win in the courtroom, he may believe that an appeal will help him politically.

Stanley Renshon, a political science professor of The City University of New York, doesn't think it would.

Renshon said a legal fight might only further anger Craig's constituents, who already believe the arrest has messed up the job he was elected to do, and who would now be asked to trust him again.

"I don't know legally whether it will do anything for him," Renshon said. "Politically, he's dead."