A House Judiciary subcommittee opened a new offensive against the Bush administration Tuesday, by authorizing a subpoena for the vice president’s chief of staff and holding the first in a series of hearings on Justice Department legal opinions regarding harsh interrogation techniques.

The panel’s Constitution, Civil Rights and Civil Liberties Subcommittee voted to authorize a subpoena for David Addington, who served as Vice President Dick Cheney’s legal adviser before becoming chief of staff in 2005, in an effort to have him appear at a future hearing, perhaps before the full committee.

The subcommittee was also prepared to authorize a subpoena for John Yoo, a former Justice Department official involved in writing the controversial memoranda. But committee aides said that Yoo has now agreed to testify at a subsequent hearing.

Democrats are trying to unearth as much information as they can about the provenance and rationale of a series of department memoranda on the legality of harsh questioning of detainees held by the United States in prisons around the world.

The administration says the techniques were necessary to protect the country from another terrorist attack. Critics say they amount to torture that is illegal under U.S. and international law, and they have charged that the legal memos were drafted as little more than cover for use of the controversial tactics. Some of those memos were later disavowed by the Justice Department.

Underlying the issue is a Democratic desire to raise new questions about President Bush’s robust view of his constitutional authority. In a series of upcoming hearings, Democrats plan to spotlight the interrogation issue in order to turn up the political heat on the administration’s broad interpretation of executive power.

House Judiciary Chairman John Conyers Jr., D-Mich., has expressed concern “that some in the administration view the President’s power as that of an imperial presidency — not a democracy.” Conyers has said the committee will be looking at “whether we need to write stronger laws to prevent a future imperial presidency from steamrolling Congress and the American people.”

Subcommittee Chairman Jerrold Nadler, D-N.Y., echoed those thoughts.

“We’ve got to constrain the power of the executive to ignore the law,” he said.

Addington had refused to testify voluntarily, and it’s still not clear whether he will. In an April 18 letter, Cheney’s office questioned whether lawmakers have the power to require him to appear, but in a May 1 letter said Addington is “prepared to accept timely service” of a subpoena for testimony while “reserving all legal authorities, immunities, questions and privileges, including with respect to the lawfulness of the inquiry.” That could mean that Addington will appear but refuse to answer many questions.

Besides Addington and Yoo, the Judiciary panel also wants to hear from former Attorney General John Ashcroft; former CIA director George Tenet; former Undersecretary of Defense Douglas J. Feith; and Daniel Levin, a former Justice Department official. Ashcroft, Feith and Levin have agreed to appear, and Tenet is in negotiations with committee staff. It is not clear when subsequent hearings will be held.

Republicans on the subcommittee indicated Tuesday that they regard the whole effort as a political charade. Trent Franks of Arizona, the top Republican on the subcommittee, played an audio clip of New York Democratic Sen. Charles E. Schumer at a Senate Judiciary hearing in 2004 in which Schumer said, in part, “It’s easy to sit back in the armchair and say that torture can never be used. But when you’re in the foxhole, it’s a very different deal.”

Franks said that harsh techniques were used as part of “efforts to save thousands of innocent American lives.”

Some of the witnesses at the hearing Tuesday said that high-level administration officials could be liable for war crimes prosecutions under domestic and international law.

“War crimes were committed,” said Philippe Sands, a University of London law professor.

Marjorie Cohn, a professor at the Thomas Jefferson School of Law, said, “Torture does not work,” and added that less coercive interrogation methods produce better results.

Franks countered that it was “naive” to think that suspected terrorists should be questioned only without using harsh methods. And another witness, former Justice Department official David Rivkin, said he thought “it would be madness to prosecute anybody given the facts involved.”

It is too early to tell whether the committee probe will result in legislation either this year or after Bush leaves office. He is likely to veto any new legislative constraints on detainee interrogations.

Democrats in the Senate want to include language in the fiscal 2009 intelligence authorization measure that would limit the government’s interrogation techniques to those approved in a September 2006 Army field manual.

That would outlaw a number of controversial tactics, such as waterboarding, a form of simulated drowning. But a similar proposal led Bush to veto the fiscal 2008 version (HR 2082).

Tim Starks contributed to this story.