At the argument in the McGahn case, Mr. Mooppan, the administration’s lawyer, made two basic arguments, both sweeping. Mr. Trump and his close advisers, Mr. Mooppan said, have absolute immunity from congressional subpoenas, meaning they cannot be made to appear to be questioned about anything at all, whether or not executive privilege or some other protection might apply to particular inquiries.

But Mr. Mooppan said there was no need for the appeals court to decide that issue, as federal courts have no role in adjudicating disputes between the other branches. In 1997, he noted, the Supreme Court rejected a suit against executive branch officials from six individual lawmakers, saying they had not suffered the sort of direct injury that gave them standing to sue.

The D.C. Circuit, however, has said that “the mere fact that there is a conflict between the legislative and executive branches over a congressional subpoena does not preclude judicial resolution of the conflict.” It drew support from United States v. Nixon, the 1974 Supreme Court decision requiring President Richard M. Nixon to turn over tapes of conversations with aides, calling that “an analogous conflict between the executive and judicial branches.”