Invoking a privilege is one thing, but telling a person not to show up in response to a subpoena -- if only to actually invoke the privilege -- is quite another. It's not just worse, it's a felony under federal criminal law. See for yourself.p.s. given the ponderous profundity of my professional pursuits, the occasion seldom arises in which occurs to me such a passing trifle as the present consideration that a rather humorous moniker for the current US president might be "Felonious Monkey."
18 U.S.C. Sec. 1505 : ... Whoever corruptly ... influences, obstructs, or impedes ... the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress ... [s]hall be fined under this title, [or] imprisoned not more than 5 years ... or both.18 U.S.C. Sec. 1515(b): As used in section 1505, the term "corruptly" means acting with an improper purpose, personally or by influencing another, including ... withholding, [or] concealing ... information.
UPDATE (12 July):
Cass Sunstein, writing in the Boston Globe, suggests that legal interpretation on the matter of executive privilege is a wash. That is, despite any appearances of political impropriety we might wish resolved, there are legal cases to be made on both the president's case for executive privilege and Congress' case for disclosure. Forcing the case in court would be the decisive factor, perhaps, but it appears that we wouldn't have any inkling of which direction the decision would go until we had the decision in our hands. This strikes me as odd in this particular case, at least. This is a country legally unprepared for a presidential abuse of power, even when most of us recognize it as abuse. We ought to question the broad assumption that law ought to trump ethics, especially given the dependence of law that is genuine in spirit if not in letter on ethical reasoning (but not necessarily the other way around).
The president's best answer is that this is not a judicial proceeding and that Congress has not demonstrated anything like sufficient need for these materials. The Department of Justice argues that Congress has received "thousands of documents and dozens of hours of testimony already." The department adds that Congress must do more than to say, in a general way, that it fears wrongdoing or that the materials "are of public import"; it must show that they are "demonstrably critical" to Congress' effort to exercise its constitutional role.
Congress' strongest reply is that the evidence reveals a real need for the documents, which are indispensable to establish whether a genuine misuse of executive power has occurred. Congress might add that its own lawmaking prerogatives are at stake. If partisan politics has affected the decision to replace US attorneys, it might seek to enact corrective legislation.
In arguing for inherent authority to engage in torture, and for the power to make war without congressional approval, the Bush administration has made some extravagant arguments about executive power. But the president's authority to resist congressional subpoenas has never been well defined. If the issue gets to court, anything can happen.