Lawyers for a former White House official charged with obstructing a leak investigation, I. Lewis Libby, are accusing the special prosecutor who brought the case, Patrick Fitzgerald, and a former top Justice Department official, James Comey, of revising their views of Mr. Fitzgerald’s authority in order to defeat the defense’s arguments that the prosecutor was unconstitutionally appointed.

In a filing on Friday, lawyers for Mr. Libby, a former chief of staff to Vice President Cheney, urged Judge Reggie Walton to discount affidavits Messrs. Comey and Fitzgerald submitted recently arguing that Mr. Fitzgerald was under Justice Department supervision and was obligated to follow Justice Department policies.

“The court should reject the government’s attempt to defend Mr. Fitzgerald’s appointment by changing the factual record in this manner,” Mr. Libby’s lawyers wrote. They have argued that the “special prosecutor” arrangement is unconstitutional because Mr. Fitzgerald was not confirmed by the Senate in that capacity. “The legality of the special counsel’s appointment should depend on the objective terms of Mr. Comey’s action, not on undisclosed subjective understandings offered for the first time in response to a legal challenge,” the defense contended.

Last month, the defense team filed with Judge Walton a list of potential trial witnesses. It included a former secretary of state, Colin Powell; a former deputy to Mr. Powell, Richard Armitage; President Bush’s top political adviser, Karl Rove; a former director of central intelligence, George Tenet; a former White House press secretary, Ari Fleischer; a former undersecretary of state, Marc Grossman; the national security adviser, Stephen Hadley, and a former CIA spokesman, William Harlow. Mr. Libby’s team said they also expected testimony from a former CIA agent whose exposure prompted the probe,Valerie Wilson, and her husband, Joseph Wilson, as well as two CIA officials whose identities remain unclear.

A trial in the case is set for January 2007.