Perspective

Atrios offers some perspective on invoking presidential privilege. Article 3 of the Nixon Articles of Impeachment:

Article 3

In his conduct of the office of President of the United States, Richard M. Nixon, contrary to his oath faithfully to execute the office of President of the United States and, to the best of his ability, preserve, protect, and defend the Constitution of the United States, and in violation of his constitutional duty to take care that the laws be faithfully executed, has failed without lawful cause or excuse to produce papers and things as directed by duly authorized subpoenas issued by the Committee on the judiciary of the House of Representatives on April 11, 1974, May 15, 1974, May 30, 1974, and June 24, 1974, and willfully disobeyed such subpoenas. The subpoenaed papers and things were deemed necessary by the Committee in order to resolve by direct evidence fundamental, factual questions relating to Presidential direction, knowledge or approval of actions demonstrated by other evidence to be substantial grounds for impeachment of the President. In refusing to produce these papers and things Richard M. Nixon, substituting his judgment as to what materials were necessary for the inquiry, interposed the powers of the Presidency against the the lawful subpoenas of the House of Representatives,
thereby assuming to himself functions and judgments necessary to the exercise of the sole power of impeachment vested by the Constitution in the House of Representatives.

In all of this, Richard M. Nixon has acted in a manner contrary to his trust as President and subversive of constitutional government, to the great prejudice of the cause of law and justice, and to the manifest injury of the people of the United States.

Issuing subpoenas is well within the power of the House. Ignoring those subpoenas has consequences. Interestingly, a post linked at the Atrios post considers the impeachment of Gonzales himself.