PURGEGATE AND EXECUTIVE PRIVILEGE….I’ve long found it vaguely surprising that the scope (and limits) of executive privilege have never been spelled out in much detail by the courts. It’s not as if the question doesn’t come up frequently, after all, and it’s not as if it isn’t a perfect, juicy topic for the judiciary to address. The president and his immediate staff really do have a strong interest in their ability to receive candid, provocative advice, and that interest is threatened if advisors are worried that the ideas they toss around in private are likely to become public. This is an important principle regardless of who occupies the White House.

Likewise, however, Congress has a strong interest in executive branch oversight, and their interest is threatened if presidents routinely refuse to allow testimony by White House aides for any reason whatsoever.

So how does executive privilege play out in Purgegate? Today in Slate, Walter Dellinger and Christopher Schroeder run down the meager judicial record on the subject and come to the following conclusion:

Communications among senior White House staff members, and between them and the president, ought to remain confidential where the only charges being investigated concern “mere” patronage appointments to the U.S. attorney posts….For these allegations, Congress can rely upon compelled testimony from Justice Department officials and others outside the White House, and voluntary testimony and evidence from within the White House.

But if there is a plausible basis for believing that the Bush administration replaced any U.S. attorney to improperly obstruct a criminal investigation or improperly prompt an indictment, or a plausible basis for believing that earlier congressional inquiries were wrongfully impeded, then claims of executive privilege should give way for evidence pertinent to that charge.

As Dellinger and Schroeder say, there’s an important distinction here: Congress is free to compel testimony from Justice Department officials regardless of the subject, but they can compel testimony from White House aides only if there’s serious reason to believe there’s been criminal wrongdoing. And — so far, at least — the only plausible issue of criminal wrongdoing involves the possibility that U.S. Attorneys were fired in order to impede or encourage partisan investigations in some way. Until and unless there’s more concrete evidence of that, Karl Rove & Co. are — unfortunately — probably justified in refusing to testify under subpoena.