Under the Constitution it is not within the power of the President to conduct an inquiry into his own impeachment, to determine which evidence, and what version or portion of that evidence, is relevant and necessary to such an inquiry. These are matters which, under the Constitution, the House has the sole power to determine.27
Consistent with that long-settled understanding, other Presidents have recognized that they must comply with information requests issued in a House impeachment inquiry. In 1846, for example, President James Polk stated in a message to the House:
It may be alleged that the power of impeachment belongs to the House of Representatives, and that with a view to the exercise of this power, that House has the right to investigate the conduct of all public officers under the government. This is cheerfully admitted. In such a case, the safety of the Republic would be the supreme law; and the power of the House in the pursuit of this object would penetrate into the most secret recesses of the executive departments. It could command the attendance of any and every agent of the government, and compel them to produce all papers, public or private, official or unofficial, and to testify on oath to all facts within their knowledge.28
Past Presidents have also produced documents and permitted senior officials to testify in connection with other Congressional investigations, including inquiries into Presidential actions.
For example, in the Iran-Contra inquiry, President Ronald Reagan's former National Security Advisor, Oliver North, and the former Assistant to the President for National Security Affairs, John Poindexter, testified before Congress.29 President Reagan also produced "relevant excerpts of his personal diaries to Congress."30
During the Clinton Administration, Congress obtained testimony from top advisors to President Clinton, including Chief of Staff Mack McLarty, Chief of Staff Erskine Bowles, White House Counsel Bernard Nussbaum, and White House Counsel Jack Quinn.31
Similarly, in the Benghazi investigation, led by Chairman Trey Gowdy, President Barack Obama made many of his top aides available for transcribed interviews, including National Security Advisor Susan Rice and Deputy National Security Advisor for Strategic Communications Benjamin Rhodes.32 The Obama Administration also produced more than 75,000 pages of documents in that investigation, including 1,450 pages of White House emails containing communications of senior officials on the National Security Council.33
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