WASHINGTON, Feb. 11 — One figure has dominated the trial of I. Lewis Libby Jr. without even showing up in the courtroom. Day after day, the jury has heard accounts of the actions of Vice President Dick Cheney, watched as his handwritten notes were displayed on a giant screen, heard how he directed leaks to the news media and ordered the White House to publicly defend Mr. Libby, his top aide and close confidante.

Now, as the defense phase of the perjury trial begins, Mr. Cheney is expected to make a historic appearance on the witness stand. It is an act of loyalty that carries considerable risk for Mr. Cheney, a powerful figure in the administration who has in recent months suffered a series of major political and policy setbacks.

If he testifies, Mr. Cheney will bring to the jurors the awesome authority of his office and could attest to Mr. Libby’s character as policy adviser and family man, and to his crushing workload and dedication to keeping the country safe. That could give extra heft to Mr. Libby’s defense against the charge that he lied to the Federal Bureau of Investigation and the grand jury: that he was so occupied with important matters of state, he did not accurately remember conversations from July 2003.

But the first 10 days of testimony have already exposed some of the long-hidden workings of Mr. Cheney’s extraordinary vice presidency, revealing how deeply Mr. Cheney himself was engaged during 2003 in managing public relations as the administration’s case for war came under attack.