Can President Trump be compelled to testify in response to a criminal subpoena and, even more consequentially, can a sitting president be criminally indicted? In a confidential letter for the special counsel, Robert Mueller, Mr. Trump’s lawyers lay out an aggressive defense against the special counsel’s pursuit of the president’s testimony. Their answer to first of those questions is a categorical no.

The letter is more monarchal than respectful of the separation of powers and our democracy’s constitutional checks and balances. Its claims that the president can “order the termination of an investigation by the Justice Department or F.B.I. at any time and for any reason” is unprecedented and far exceeds even Harry Truman’s brazen and rejected attempt to take over the steel mills to blunt labor unrest in the 1950s.

In fact, the letter’s absence of meaningful constitutional inquiry beyond baldfaced claims that Article II establishes a unitary executive gives the impression that these questions have gone unexamined.

But many Americans have wrestled with these questions. In the late 1980s, I headed the Office of Legal Counsel, an elite division within the Department of Justice. In 1988, with a White House entangled by Iran-contra, the O.L.C. was asked whether a president could be ordered to comply with a criminal subpoena seeking his testimony. We also responded in the negative — but it was not a simple, categorical no. The presidential subpoena is a valid legal tool, as Chief Justice Warren Burger made clear in United States v. Nixon, but a president may find case-specific reasons to resist it.