Some Senate Democrats say Mr. Bush is just "stonewalling" and insinuate that he must be trying to hide something, as Judiciary Committee Chairman Patrick Leahy (D., Vt.) has darkly intoned. But as he well knows, executive privilege traces its lineage to George Washington. In 1796, the House of Representatives demanded all his papers related to the controversial Jay Treaty with Great Britain. Washington refused, saying that the Constitution barred the House from the making of treaties. Firing U.S. attorneys and any other executive officers, including those requiring Senate approval, rests beyond the constitutional powers of Congress, and totally within those of the presidency. This has been true since the first cabinet departments were established in 1789.
The Supreme Court held in 1959 that, "Since Congress may only investigate into those areas in which it may potentially legislate or appropriate, it cannot inquire into matters which are within the exclusive province of one or the other branches of the Government." In the 1974 Watergate tapes case, the Supreme Court said that the president's right to protect information is strongest when law enforcement, national security or his other constitutional powers are involved. Under that rule, Mr. Leahy has no right to see the president's communications about the firing of federal attorneys, the nomination of John Roberts or Samuel Alito to the Supreme Court or the reduction of Scooter Libby's sentence.
Since the Justice department was created by congress as was the office of U.S. attorney I would say that does fall in their area. For that matter they could end the justice department and set up a different law enforcement office.
Pardons are clearly a constitutional power of the President - that’s different. Not Congress’ area.
BTW, that George Washington case was resolved without the courts when he gave the documents to the senate.
The Thomas Jefferson case where his vice President was being investigated went to the supreme court and Jefferson lost the privilige argument. “As for Jefferson’s claim that disclosure of the document would imperil public safety, Marshall held that the court, not the president, would be the judge of that. Jefferson complied with Marshall’s order.”