President Bush has commuted the sentence of a former White House aide, I. Lewis Libby Jr., who was facing 2 1/2 years in prison for obstructing an investigation into the leak of a CIA officer’s identity.

“I respect the jury’s verdict. But I have concluded that the prison sentence given to Mr. Libby is excessive,” Mr. Bush said in a written statement this afternoon. “Therefore, I am commuting the portion of Mr. Libby’s sentence that required him to spend thirty months in prison.”

Mr. Bush acted on the same day that a federal appeals court panel unanimously rejected Libby’s bid to remain free while he appealed his convictions for obstruction of justice, false statements, and perjury.

If Mr. Bush had not intervened, Libby would likely have been required to report to prison within the next few weeks. The president did not disturb a $250,000 fine and two years of probation that the trial judge, Reggie Walton, also imposed.

“My decision to commute his prison sentence leaves in place a harsh punishment for Mr. Libby,” Mr. Bush said. “The reputation he gained through his years of public service and professional work in the legal community is forever damaged. His wife and young children have also suffered immensely. He will remain on probation. The significant fines imposed by the judge will remain in effect. The consequences of his felony conviction on his former life as a lawyer, public servant, and private citizen will be long-lasting.”

The three appeals court judges who considered Libby’s request for release pending his appeal, David Sentelle, Karen Henderson, and David Tatel, offered only a brief explanation of why the motion was rejected. “Appellant has not shown that the appeal raises a substantial question” under the federal bail statute, the judges’ order said. They noted that under the law a substantial question is defined as one that is “close” or “could very well be decided the other way.”

The court decision putting Libby ever closer to the jailhouse door seemed certain to step up pressure on Mr. Bush to grant a pardon or some other form of clemency. Libby has a large contingent of supporters who have asserted that he is the victim of overzealous pursuit by the special prosecutor who brought the case, Patrick Fitzgerald.

In his statement, Mr. Bush took note of various arguments made by Libby’s allies, but he did not say whether he agreed with any of them, except for the contention that Libby’s sentence was too severe. The president explicitly rejected criticism that has been leveled at the special prosecutor. “Mr. Fitzgerald is a highly qualified, professional prosecutor who carried out his responsibilities as charged,” Mr. Bush said.

Libby’s full appeal of his convictions could continue and may take another year or more. In recent days, aides to Mr. Bush said he would not consider any pardon until the legal process was complete. However, a statement last month from a departing White House aide, Daniel Bartlett, suggested presciently the president might take up the issue of clemency if Libby’s jailing seemed imminent.

Mr. Fitzgerald, who was appointed to investigate the leak to the press of the identity of a CIA operative, Valerie Plame, never brought charges against anyone for leaking. However, Libby was charged with obstructing the probe by lying about where and when he learned of Ms. Plame’s CIA tie. A jury convicted Libby in March on one count of obstruction of justice, two counts of lying to a grand jury, and one count of lying to the FBI.

Libby argued to Judge Walton that he should be released during his appeal because he had strong legal issues to raise. The most notable was that Justice Department officials gave Mr. Fitzgerald so much authority that he was constitutionally unaccountable. Judge Walton said he found that argument unpersuasive. Now, the appeals court seems to have agreed.

The appeals panel’s ruling was also something of a snub to a dozen eminent legal scholars, including a Harvard law professor, Alan Dershowitz, and a former federal appeals court judge, Robert Bork. They attempted to file with the court an amicus brief contending that Libby had valid arguments against the constitutionality of Mr. Fitzgerald’s appointment and conduct in office. The appeals panel refused the brief and, today, suggested that those issues are less substantial than the professors contended.