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.’ ”

As a purely legal matter, of course, Mr. Bush’s statement has no particular force outside Mr. Libby’s case. But that does not mean judges will necessarily ignore it.

No one disputes that Mr. Bush has the authority under the Constitution to issue pardons and commutations for federal crimes. But experts in the area, pointing to political scandals in the Reagan, Truman and Grant administrations, said Mr. Bush had acted with unusual speed.

“What distinguishes Scooter Libby from the acts of clemency in the other three episodes,” said P. S. Ruckman Jr., a political science professor who studies pardons at Rock Valley College in Rockford, Ill., referring to Mr. Libby by his nickname, “is that in those episodes they generally served their time and some other president pardoned them.”

Mr. Bush repeated yesterday that he had found Mr. Libby’s punishment to be too severe. But experts in federal sentencing law said a sentence of 30 months for lying and obstruction was consistent with the tough sentences routinely meted out by the federal system.

“On what legal basis could he have reached that result?” asked Frank O. Bowman III, an authority on federal sentencing who teaches law at the University of Missouri-Columbia, said of the commutation. “There is no legal basis.”

Nor is there a reason to think that the Justice Department has changed its position about the sentencing system generally. Indeed, Attorney General Alberto R. Gonzales said last month that the department would push for legislation making federal sentences tougher and less flexible.