The Mueller report set forth a compelling case that President Donald Trump obstructed justice. And Democrats appear to believe the star witness in their obstruction investigation will be former White House Counsel Don McGahn, who memorably told his colleagues the president was trying to get him to do “crazy s–t.” The House Judiciary Committee has subpoenaed McGahn to testify next month, no doubt hoping that he will deliver more damning anecdotes about his former boss on live television.

But McGahn’s testimony, assuming the White House doesn’t succeed in blocking it, may not end up helping Democrats. In fact, if they don’t prepare as if they were going to trial—in short, treat McGahn like an adverse witness—it could actually weaken any case for impeachment they might decide to bring later.

That’s why my first goal if I were advising Congress would be to hold off on the subpoena until Attorney General William Barr was forced to supply the entire, unredacted report from the special counsel and all of the underlying investigative materials, including the FBI reports of McGahn’s interviews.