Vice President Cheney will be called as a defense witness at the upcoming trial of his former chief of staff, I. Lewis Libby, on obstruction of justice and perjury charges stemming from the CIA leak case, an attorney said yesterday.

“We’re calling the vice president,” one of Mr. Libby’s lawyers, Theodore Wells Jr., said during a pre-trial hearing in federal court in Washington, according to the Associated Press. Another attorney for Mr. Libby, William Jeffress, said he did not expect the vice president to resist the request that he testify at the trial, which is scheduled to begin January 16.

“We’ve cooperated fully in this matter and we’ll continue to do so,” a spokeswoman for Mr. Cheney, Lea Anne McBride, said in a statement. “In fairness to the parties involved and, as we’ve stated previously, we’re not going to comment further on a legal proceeding.”

Mr. Libby resigned last year after being indicted for lying to investigators and a grand jury exploring the disclosure of the identity of a CIA operative, Valerie Plame, in the press in 2003. Suspicions about the leaks centered on White House officials because Ms. Plame’s husband, Joseph Wilson, had publicly challenged President Bush’s statements about Iraq’s efforts to obtain nuclear materials in Africa.

No one involved was ever charged with violating the law by leaking. It has since been established that the first person to discuss Ms. Plame’s CIA employment with a reporter was the deputy secretary of state at the time, Richard Armitage, who was not particularly close to White House officials.

However, the special prosecutor assigned to the case, Mr. Fitzgerald, sought charges against Mr. Libby for claiming to have learned of Ms. Plame’s CIA affiliation from the press. In fact, the prosecutor said, the White House aide deliberately obscured the fact that he was told about her link by government officials and later shared that fact with the press.

Mr. Libby is fighting the charges, contending that any inaccurate statements he made were due to memory failure brought on by the volume of urgent national security matters he dealt with.

Mr. Cheney will hardly be a hostile witness for the defense. In a June interview with CNN, he called Mr. Libby “one of the finest men I’ve ever known.”

The vice president has already been featured prominently in the pre-trial motions related to the case. One filing disclosed that Mr. Cheney scrawled questions about Mr. Wilson onto a New York Times op-ed the former ambassador wrote just before his wife was allegedly exposed. Another brief said Mr. Cheney got Mr. Bush’s personal approval to declassify a report on Iraq that Mr. Libby provided to selected journalists.

Historians and legal scholars described the expected testimony as highly unusual. “It is very rare for a sitting president or vice president to testify in any setting,” a law professor at Ohio State University, Peter Shane, said.

While in office, President Clinton testified, via video link, to a grand jury investigating the Monica Lewinsky scandal, a flap that stemmed, in turn, from a deposition Mr. Clinton gave in a civil case.

After leaving office, President Reagan testified on videotape for the trial of one of his national security advisers, Admiral John Poindexter.

While still in office, President Ford testified voluntarily before Congress about his pardon of President Nixon.

Mr. Cheney’s willingness to testify without a legal fight is notable because he has been an ardent defender of presidential prerogatives and has often warned of the dangers posed by the erosion of executive privilege.

“That is seemingly inconsistent, if not with the legal positions he has taken in other instances, certainly with the normal attitude of the vice president,” Mr. Shane said. However, he added that any resistance might have been futile because legal precedents give criminal defendants strong authority to call witnesses, even those who work at the White House.

The slippery slope Mr. Cheney has alluded to was apparent immediately yesterday as the Web site of a liberal magazine, the Nation, argued that the vice president’s plan to appear at the trial obligates him to testify before Congress on related issues.

“If the vice president is willing to testify in Libby’s trial, then surely Congress has not just the right but the Constitutional duty to suggest that Cheney must also take questions from the Congress,” a Web logger for the Nation, John Nichols wrote.

About a year ago, Rep. Maurice Hinchey of New York and two Democratic colleagues asked Mr. Cheney to testify before Congress about the leaks relating to Ms. Plame. Starting next month, Democrats will have the power to subpoena the vice president, if they choose to.

In an interview just before last month’s election, Mr. Cheney said he would “probably not” comply with any congressional subpoena. “The president and the vice president are constitutional officers and don’t appear before the Congress,” he told ABC.