WASHINGTON -- Risking a political backlash, President Bush yesterday threw out the 2 ¹/_2-year prison sentence for former White House aide I. Lewis "Scooter" Libby, but stopped short of giving him a full pardon.

The commutation pleased Mr. Libby's conservative allies, but Democrats accused Mr. Bush of trampling the rule of law. Mr. Libby, who was convicted of perjury and obstruction of justice in the CIA leak case, still faces a $250,000 fine and two years' probation. Unlike a pardon, a commutation does not forgive the underlying offense.

"I respect the jury's verdict. But I have concluded that the prison sentence given to Mr. Libby is excessive," Mr. Bush said. The president spared Mr. Libby hours after a federal appellate court refused to let Mr. Libby remain free while he appealed his conviction.

Mr. Bush ignored the usual process for commutations, which calls for the Justice Department to review the case after the convicted felon reports to prison. The department's guidelines say commuting a sentence is "an extraordinary remedy that is rarely granted."

Democrats expressed outrage. "Until now, it appeared that the president merely turned a blind eye to a high-ranking administration official leaking classified information. The president's action today makes it clear that he condones such activity," said House Judiciary Committee Chairman John Conyers, D-Mich. "This decision is inconsistent with the rule of law and sends a horrible signal to the American people and our intelligence operatives who place their lives at risk every day."

Republicans who had called for a full pardon welcomed Mr. Bush's decision to ease the punishment. The president could still pardon Mr. Libby if his appeals fail.

"The betting line has to be that the president will pardon Scooter Libby on his last day in office," Republican commentator Rich Galen said. "I thought he should have pardoned him last January, before the trial."

Still, it wasn't clear whether the commutation would have much political impact. Some analysts predicted that the decision simply reinforced well-established opinions about the president: Critics have another reason to dislike him; admirers have another reason to support him.

"It's a way for him to unite the troops -- his political allies or people who should be his political allies," said Drake University political science professor Dennis Goldford in Des Moines, Iowa.

Mr. Bush acknowledged the seriousness of the charges against Mr. Libby, who was convicted of lying to investigators trying to determine who leaked the identity of CIA officer Valerie Plame. Ms. Plame and her husband, former Ambassador Joseph Wilson, contend that she was exposed in retaliation for her husband's public criticism of the Iraq war.

Testimony in the case showed that Mr. Libby wasn't the original leaker, but he did discuss Ms. Plame's CIA ties with reporters. The judge and the jury agreed with prosecutors that he lied repeatedly to investigators.

"If a person does not tell the truth, particularly if he serves in government and holds the public trust, he must be held accountable," Mr. Bush said. "My decision to commute his prison sentence leaves in place a harsh punishment for Mr. Libby. The reputation he gained through his years of public service ... is forever damaged. His wife and young children have also suffered immensely."

Mr. Bush had vowed at the start of the investigation in 2003 to take a hard line against anyone involved in the leak. He ordered his staff to cooperate with investigators and threatened to fire the leaker.

"If there is a leak out of my administration, I want to know who it is. And if the person has violated law, the person will be taken care of," he said then. "I want to know the truth."

And he has repeatedly refused to discuss the possibility of a pardon.

Vice President Dick Cheney, Mr. Libby's chief benefactor in the White House, issued a statement praising his former aide, without commenting directly on the commutation. He called Mr. Libby, who served as his chief of staff as well as an adviser to Mr. Bush, a person of integrity.

Federal prosecutor Patrick Fitzgerald, the chief investigator in Mr. Libby's case, had argued for a stiff prison sentence for the defendant. "Mr. Libby, a high-ranking public official and experienced lawyer, lied repeatedly and blatantly about matters at the heart of a criminal investigation concerning the disclosure of a covert intelligence officer's identity," Fitzgerald said during the trial's sentencing phase last May. "He has shown no regret for his actions, which significantly impeded the investigation."

In a statement last night, Mr. Fitzgerald took issue with Mr. Bush's characterizing of the sentence as "excessive." The prosecutor said: "We fully recognize that the Constitution provides that commutation decisions are a matter of presidential prerogative, and we do not comment on the exercise of that prerogative.

"... The sentence in this case was imposed pursuant to the laws governing sentencings, which occur every day throughout this country. In this case, an experienced federal judge considered extensive argument from the parties and then imposed a sentence consistent with the applicable laws."

Mr. Fitzgerald said he would continue to "seek to preserve [Libby's] convictions through the appeals process."

Other presidents also have issued commutations in politically charged cases. Former President Bill Clinton's last-minute clemency orders included a commutation for former Rep. Melvin Reynolds of Chicago, who was released early from his 6 ¹/₂-year sentence on fraud and corruption charges.

Mr. Libby was the highest-ranking sitting administration official to be indicted in decades. The last was in 1984, when then-President Ronald Reagan's labor secretary, Raymond Donovan, was charged with grand larceny, but acquitted.