"Special Counsel Robert Mueller reportedly warned President Trump's lawyers in a March meeting that if the President declined to participate in a voluntary interview, Mueller could issue a subpoena compelling the President's testimony before a grand jury. The alleged exchange raises the question of whether a sitting President, consistent with the separation of powers and Article II of the Constitution, may be required to comply with a subpoena for his testimony as part of an ongoing criminal investigation. Before addressing this question, it is necessary to define the type of subpoena that may be envisioned by the Special Counsel. Special Counsel Mueller, who has been vested with the powers and duties of 'any United States Attorney,' has not been provided independent and unilateral authority to issue generalized subpoenas for testimony. Instead, any subpoena to the President would likely be issued by a grand jury-- at the request of the Special Counsel--but under the authority of the judiciary. The subpoena would therefore accurately be framed as an attempt by the judicial branch to compel testimony from the President, giving rise to possible concerns under the separation of powers."
CRS Legal Sidebar, LSB10130
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html