Lawyers for a former White House aide facing 2 1/2 years in prison, I. Lewis Libby Jr., have made their final written plea to a federal appeals court panel, which could save him from having to begin his sentence in the coming weeks.

In a brief made public yesterday, Libby’s attorneys press their argument that the broad authority that Justice Department officials granted to the special prosecutor, Patrick Fitzgerald, violated the Constitution’s requirement that principal officers in the executive branch be nominated by the president and confirmed by the Senate.

“Neither Judge Walton nor the Special Counsel have explained how the after-the-fact, all-or-nothing power to remove, unless coupled with the right to be informed, could constitute ‘direction and supervision,'” the defense lawyers wrote, responding to arguments that Mr. Fitzgerald was subject to being fired at any time by a top Justice Department official. “If Special Counsel Fitzgerald was removable at all, it was by someone who would never learn important details of what he was doing in the first place.”

A jury convicted Libby in March on charges of obstruction of justice, perjury, and lying to the FBI during an investigation into leaks of the identity of a CIA operative, Valerie Plame.

The latest filing takes a more confrontational tack against the trial judge in the case, Reggie Walton. He filed a 30-page opinion last week bolstering his refusal to delay the start of Libby’s prison term until after his appeals are resolved.

The defense team also noted that Mr. Fitzgerald signed a certification that disclosing that certain classified information could harm national security, even though he was not permitted to do so under the law. The prosecutor and Judge Walton said the error was harmless, but the defense contends that it underscores their point that no one was supervising Mr. Fitzgerald. “If Fitzgerald was not a ‘principal’ officer at that moment, who exactly was?” the defense attorneys asked.

Three judges from the D.C. Circuit Court of Appeals, David Sentelle, Karen Henderson and David Tatel, are expecting to decide soon whether Libby’s arguments on appeal have a substantial chance of overturning his conviction. If so, he is entitled to a stay of his sentence. If not, he must report to prison.

If Libby is ordered to report to prison immediately, his supporters are expected to step up pressure on President Bush to grant a pardon or some other form of clemency, which could keep the former chief of staff to Vice President Cheney out of jail.