House Democrats are setting an unwise precedent by asking a federal judge to force testimony by former White House counsel Don McGahn as part of their fevered attempt to prove that President Trump committed impeachable offenses.

The Democrats’ request raises significant separation of powers concerns.

It is axiomatic that attorney-client privilege is a nearly sacrosanct part of the U.S. justice system. It is true that such confidentiality can, in extraordinarily rare instances, be breached by order of a court. Emphasis on “extraordinarily rare.” Among other things, the privilege is a crucial aspect of the Constitution’s protection against forced self-incrimination.

In the case of the White House counsel, the client is the presidency as an institution. This makes a legislative attempt to breach the privilege even more worrisome. Not only is attorney-client privilege at issue, but so is “executive privilege," a less sacrosanct, but still important, protection of the Constitution’s delegation of distinct powers to distinct branches of government, even in a system where some powers overlap.

If there is any single member of the White House staff whose confidential discussions with the president should be protected from being pierced by legislative inquiry, it is the White House counsel. Nobody else combines both privileges of confidentiality in such a strong fashion. It is especially in the realm of legal advice that the executive branch of government rightly enjoys an expectation of protection against legislative encroachments.

The counterargument, however, says that McGahn already has testified to the special counsel, and that the House effort is merely an extension of that process. This counterargument is dead wrong. With the president’s explicit permission, McGahn spoke with the special counsel, as one member of the executive branch to another member of the executive branch who formally was under the president’s nominal control. The special counsel was conducting an inquiry under executive authority, one whose proceedings by law could have been kept from public view.

No issues of separation of powers applied there.

For the House now to approach the third branch of government, the judiciary, to compel a breach of not one but two rightful privileges enjoyed (again, with rare exceptions) by the second branch, is for the House to tread on shaky ground indeed.

If a Republican House had tried to do this to President Barack Obama, the same House Democrats now pushing for McGahn’s testimony would have been yelling bloody murder against the effort.

If House Democrats truly believe Trump obstructed justice, they have every right to conduct an inquiry and every right to cite the substance of McGahn’s interviews with the special counsel in doing so. To compel more testimony than that, though, is to undermine the Constitution in the name of serving it. The judge ought to quash their political bloodlust, and quickly.