United States v. Nixon

...the Supreme Court in United States v. Nixon (1974) held that executive privilege cannot be invoked at all if the purpose is to shield wrongdoing.

Heritage Foundation:
Fast And Furious: Executive Privilege Is Illegitimate to Shield Wrongdoing

...The history of executive–congressional relations is filled with accommodations and waivers of privilege. In contrast to voluntary waivers of privilege, Watergate demonstrates that wrongful invocations of privilege can seriously damage the office of the presidency when Congress and the courts impose new constraints on the President’s discretion or power (some rightful and some not).

But there is at least one helpful development in Holder’s request that the President invoke executive privilege to shield these documents: The President now owns the consequences of further stonewalling. There is no ongoing DOJ prosecution or investigation to protect. There is no obvious reason why the President can’t waive even what legitimately privileged documents there are (which is probably far fewer than the 1,300 pages being withheld).

The American people will now clearly understand that it is President Obama who doesn’t want them to know who is to blame for the Fast and Furious scandal—and whether his Administration has done anything to prevent it from happening again.