President Bush used executive privilege to deny access to the testimony of two former aides who Congress wants to talk to about the firing of federal prosecutors. Headline News

the crime or act of willfully interfering with the process of justice and law esp. by influencing, threatening, harming, or impeding a witness, potential witness, juror, or judicial or legal officer or by furnishing false information in or otherwise impeding an investigation or legal process.

The courts have recognized a president’s limited right to keep the White House’s internal deliberations private. But it is far from an absolute right, and Mr. Bush’s claim of executive privilege in the attorneys scandal is especially ludicrous. The White House has said repeatedly that Mr. Bush was not involved in the firings of nine United States attorneys. If that’s true, he can hardly argue that he has the right to conceal conversations and e-mail exchanges that his aides had with one another and the Justice Department.
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If the White House continues to defy Congress, the Senate and the House could file criminal contempt charges. It’s a strong measure, but lawmakers should not be afraid to take it, as they have done 10 times since 1975 under both parties. NY Times

In the same convention George Mason argued that the President might use his pardoning power to "pardon crimes which were advised by himself" or, before indictment or conviction, "to stop inquiry and prevent detection."

James Madison responded:

[I]f the President be connected, in any suspicious manner, with any person, and there be grounds to believe he will shelter him, the House of Representatives can impeach him; they can remove him if found guilty.. Daily Kos

The President's need for complete candor and objectivity from advisers calls for great deference from the court. However, when the privilege depends solely on the broad, undifferentiated claim of public interest in the confidentiality of such conversations, a confrontation with other values arises. Absent a claim of need to protect military, diplomatic, or sensitive national security secrets, we find it difficult to accept the argument that even the very important interest in confidentiality of Presidential communications is significantly diminished by production of such material for in camera inspection with all the protection that a district court will be obliged to provide.