Special Prosecutor Patrick Fitzgerald urged a federal judge Tuesday not to delay former White House aide I. Lewis "Scooter" Libby's 2 1/2-year prison sentence in the CIA leak case.

Libby, the former chief of staff to Vice President Dick Cheney, has argued that he has a good chance of winning an appeal and should be allowed to remain free until that challenge has run its course.

U.S. District Judge Reggie B. Walton, who sentenced Libby to prison for lying to authorities and obstructing the investigation into the 2003 leak of CIA operative Valerie Plame's identity, has said he sees no reason to grant Libby's request. He did not set a date for Libby to report to prison, however, and scheduled a hearing on the issue for Thursday.

A delay in Libby's sentence would give President Bush more time to consider pardon requests from Libby's supporters, who say the loyal aide was caught up in a political investigation and does not deserve prison time.

Fitzgerald, in court documents filed Tuesday, said an appeals court is unlikely to overturn Libby's conviction because the evidence against him was so overwhelming.

After a monthlong trial, jurors found that Libby lied to investigators about how he learned that Valerie Plame, the wife of an outspoken war critic, worked for the CIA, and whom he told.

Defense attorneys contended that Fitzgerald lacked the constitutional authority to bring charges against Libby. They also said they were unfairly prohibited from discussing the classified issues that were weighing on Libby's mind at the time and from questioning NBC reporter Andrea Mitchell about why she said lots of journalists knew Plame's identity.

On the classified information issue, Fitzgerald noted that defense attorneys did not call Libby or Cheney to testify to bolster Libby's defense, despite saying they would.

If Libby's request to remain free is denied, defense attorneys will rush to an appeals court and ask judges there to put the sentence on hold.