Watergate gave executive privilege more legal heft. President Nixon asserted that he had broad powers to withhold from Congress executive branch documents, testimony from officials, and (most crucially) his White House tapes. Federal judges ruled that presidents did indeed have a presumption of executive privilege. But they also held that this power isn’t limitless. Executive privilege could be overruled if Congress showed an overriding public need for the information. In Nixon’s case, that’s what happened.
According to an authoritative Congressional Research Service history of executive privilege, at least three important elements must be present for a legally correct assertion of the power. First, the communication the president wishes to withhold must bear on a core power of the presidency, such as the right to grant pardons or conduct law enforcement. Second, the communication must have come from or to the president or a close White House adviser. Third, the communication can’t contain info so unique that investigators can’t figure it out by looking elsewhere.
In the 2009-10 “Fast and Furious” case, Arizona-based agents of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were trying to build a case against smugglers suspected of supplying violent Mexican drug cartels with weapons. The US agents allowed these suspects to purchase upwards of 2,000 guns without intervention. Some of these weapons were later implicated in a 2010 shootout that killed a federal border agent.