Perhaps inadvertently, Mr. Bush’s decision to grant a commutation rather than an outright pardon has started a national conversation about sentencing generally.

“By saying that the sentence was excessive, I wonder if he understood the ramifications of saying that,” said Ellen S. Podgor, who teaches criminal law at Stetson University in St. Petersburg, Fla. “This is opening up a can of worms about federal sentencing.”

The Libby clemency will be the basis for many legal arguments, said Susan James, an Alabama lawyer representing Don E. Siegelman, the state’s former governor, who is appealing a sentence he received last week of 88 months for obstruction of justice and other offenses.

“It’s far more important than if he’d just pardoned Libby,” Ms. James said, as forgiving a given offense as an act of executive grace would have had only political repercussions. “What you’re going to see is people like me quoting President Bush in every pleading that comes across every federal judge’s desk.”

Indeed, Mr. Bush’s decision may have given birth to a new sort of legal document.

“I anticipate that we’re going to get a new motion called ‘the Libby motion,’ ” Professor Podgor said. “It will basically say, ‘My client should have got what Libby got, and here’s why.’ ”

That Bush chose to make an exception for a political ally is galling to many career Justice Department prosecutors and other legal experts. Federal prosecutors said Tuesday the action would make it harder for them to persuade judges to deliver appropriate sentences.

The critics included some Republicans who said Bush's decision did not square with an administration that had been ardently pro law-and-order. "It denigrates the significance of perjury prosecutions," John S. Martin Jr., a former U.S. attorney and federal judge in New York, said of the commutation.

[...]

Several federal prosecutors interviewed by The Times also said they were concerned that Bush's decision would send the wrong message to judges, giving them reason to lighten sentences and undermining the goal of a more uniform justice system.

"Consistency and fidelity to the law are extraordinarily important. We have expended a lot of credibility to get judges to buy off on this," said one senior federal prosecutor who requested anonymity because he was not authorized to speak publicly about the issue.

"I don't know how I am going to advise my people," the prosecutor said. "I cannot tell you how depressed and disgusted people are around here with this decision. It really undercuts law enforcement."