A former White House aide who had his 2 and 1/2 year prison sentence for obstruction of justice commuted to no jail time by President Bush, I. Lewis Libby Jr., will be required to serve two years supervised release similar to probation, a federal judge ruled Thursday.

However, Judge Reggie Walton said he reached that conclusion "with great reservation" and he also delivered a forceful rebuttal to Mr. Bush's assertion that the sentence imposed in the case was too severe.

"It is fair to say that the Court is somewhat perplexed as to how its sentence could be accurately characterized as excessive,'" Judge Walton wrote in a footnote to his 10-page opinion upholding Libby's probation. "Although it is certainly the president's prerogative to justify the exercise of his constitutional commutation power in whatever manner he chooses (or even to decline to provide a reason for his actions altogether), the Court notes that the term of incarceration imposed in this case was determined after a careful consideration of each of the requisite statutory factors and was consistent with the bottom end of the applicable sentencing range as properly calculated under the United States Sentencing Guidelines."

The White House had no immediate comment on the ruling.

Judge Walton also suggested that Mr. Bush's contention that the sentence was too long flew in the face of the position his administration took in a Supreme Court case decided last month. In that case, a military veteran with no criminal record applicable to the guidelines, Victor Rita, was sentenced to 33 months for obstruction of justice.

Judge Walton noted that the Justice Department took the position that Rita's sentence was reasonable. The judge also noted that Attorney General Gonzales recently urged Congress to restore the sentencing guidelines to the binding status they had before the Supreme Court ruled them to be advisory in 2005.

As for the supervised release, Judge Walton said the president's authority to convert the sentence to what amounts to probation appeared to be within the scope of a 1974 Supreme Court decision, Schick v. Reed, that upheld the president's right to covert a sentence of death into life imprisonment without the possibility of parole.

As Judge Walton pointed out in an order soon after the commutation, the federal criminal statute governing supervised release dictates that it be imposed after a term of incarceration. Libby has been free on bond since his indictment in 2005. In March of this year, a jury convicted him of obstruction of justice, lying to the FBI, and perjury before a grand jury investigating the disclosure of the identity of a CIA operative, Valerie Plame. Neither Libby nor anyone else was charged with leaking Ms. Plame's CIA link.

Judge Walton concluded that Mr. Bush's action was an affront to the laws Congress has passed regarding criminal punishment, but an affront that seemed to be permitted by the Supreme Court. "The President has effectively rewritten the statutory scheme on an ad hoc basis to make the punishment created by Congress applicable to a situation Congress clearly did not intend," the judge wrote. He called the precedent in the area "unsettled" and ruled that the president's action should therefore be left in place.

Judge Walton rejected an argument by the special prosecutor, Patrick Fitzgerald, that Libby was eligible for supervised release under the letter of the law because he was in the custody of the Marshals Service while being fingerprinted and photographed. "It is difficult to imagine anyone who would be in need of assistance to transition [back] to community life' after spending several hours undergoing routine processing," the judge wrote.

Since the White House, the prosecution, and the defense all agreed that the supervised release should remain in place, no appeal on that issue seems likely.

Judge Walton was appointed by President George H.W. Bush.

Speaking a news conference today, President Bush seemed to suggest that a former State Department official, Richard Armitage, who has acknowledged being a source of the leak that led to Ms. Plame being identified publicly, should have made that fact known to the public in 2003.

"I've often thought about what would have happened had that person come forth and said, `I did it.' Would we have had this, you know, endless hours of investigation and a lot of money being spent on this matter?" Mr. Bush said.

Mr. Bush did not mention that, at that time, the FBI and the White House were instructing all government officials to refrain from public comment about the matter in order to allow the investigation to proceed.

Asked whether he was disappointed that White House officials, including Libby, also discussed Ms. Plame with the press, the president did not respond directly. "It has been a tough issue for a lot of people in the White House, and it's run its course, and now we're going to move on," Mr. Bush said.