In May 1955, McCarthy threatened to issue subpoenas to White House personnel. Eisenhower was furious, and issued an order as follows: "It is essential to efficient and effective administration that employees of the Executive Branch be in a position to be completely candid in advising with each other on official matters ... it is not in the public interest that any of their conversations or communications, or any documents or reproductions, concerning such advice be disclosed." This was an unprecedented step by Eisenhower to protect communication beyond the confines of a cabinet meeting, and soon became a tradition known as executive privilege. Ike's denial of McCarthy's access to his staff reduced McCarthy's hearings to rants about trivial matters, and contributed to his ultimate downfall.

What did McCarthy threaten to do in May of 1955?