Obama did the same for the Fast and Furious investigation....And there is legal basis for executive privilege:
The Supreme Court confirmed the legitimacy of this doctrine in
United States v. Nixon in the context of a subpoena emanating from the judiciary, instead of emanating from Congress.
[3] The Court held that there is a qualified privilege, which once invoked, creates a presumption of privilege, and the party seeking the documents must then make a "sufficient showing" that the "presidential material" is "essential to the justice of the case".
Chief Justice Warren Burger further stated that executive privilege would most effectively apply when the oversight of the executive would impair that branch's national security concerns.
[3] Regarding requests from Congress (instead of from the courts) for executive branch information, as of a 2014 study by the
Congressional Research Service,
[4] only two federal court cases had addressed the merits of executive privilege in such a context, and neither of those cases reached the Supreme Court.
[5]
Wiki
Congress is too quick to issue subpoenas based on political considerations instead of true official considerations. No branch of government can be trusted with that amount of unrestrained power and authority. The 1868 impeachment of President Johnson is an excellent case in point