From this "Review and Outlook" piece:
We hope Mr. Bush does resist, because no responsible President could allow such an intrusion into private executive branch discussions. In a June 28 response to the two chairmen, White House Counsel Fred Fielding refused to comply and invoked executive privilege, noting that the President's right to confidential internal deliberations was fundamental to his ability to perform his Constitutional duties. Mr. Bush appears ready to go to court to defend that principle, and Democrats must now decide if they want to spend years litigating a weak case, one that won't likely be settled until after Mr. Bush leaves office. These columns realize that executive privilege has its limits, and that Presidents may not use it to protect themselves from criminal or personal civil claims (read: Watergate, Paula Jones). Yet the judiciary has also affirmed a President's right to confidential internal deliberations. As Solicitor General Paul Clement noted in an advisory letter to Mr. Bush last week, one of the "underlying purposes" of executive privilege is to ensure "that senior Government officials and their advisers speak frankly and candidly during the decision-making process." That privilege seems especially strong in the U.S. attorneys case.