https://www.lawfareblog.com/congressional-subpoena-power-and-executive-privilege-coming-showdown-between-branches
If history is any guide, the question of what executive branch documents and testimony the 116th Congress will receive access to will likely not be decided by a court. Disputes between Congress and the president over the scope of executive privilege are better understood as political battles with legal underpinnings—not as matters of pure law. As such, debates over the scope of executive privilege are likely to play out in the context of negotiations between House Democrats and the Trump administration, with each side making political calculations about what fights are worth having, when it makes sense to fight and when it makes sense to cooperate.
In the long run, that’s probably a good thing—shifting resolution of interbranch privilege disputes to the courts undermines Congress’s ability to maintain its place as a coequal branch in the constitutional scheme. In the short term, however, the reality is that those eager for House Democrats to obtain information relevant to their oversight efforts in an orderly, timely and clear-cut way are likely to feel some frustration over the next two years. The White House holds a lot of cards in this game.
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A valid assertion of executive privilege provides a lawful basis to decline to answer a congressional subpoena for testimony or documents. At its most basic, executive privilege is the proposition that certain confidential or sensitive communications within the executive branch are constitutionally protected from compelled disclosure to the executive’s coequal branches—Congress and the courts. There is no mention of executive privilege in the United States Constitution—rather, it is a principle implied in the Constitution’s separation of powers. Bies identifies the five general types of executive privilege that the executive branch has claimed in the past: presidential communications, deliberative process, attorney-client communications, law enforcement investigations, and sensitive military, diplomatic and national security information. But the precise contours of any executive privilege are contested, and the executive branch, the courts and Congress tend to take divergent positions that favor their respective constitutional roles.