McGahn’s testimony could rock the White House. But it might come too late.

Remember Donald McGahn?

The House Judiciary Committee has gone to court to enforce the subpoena it issued in April for the former White House counsel’s testimony. Arguments are teed up, and the court has an expedited schedule providing for a fairly fast resolution of the dispute, with a hearing on the merits Oct. 31. Expect the courts to quickly dispose of the administration’s far-fetched argument that McGahn is “absolutely immune” from testifying, clearing the way for his critical appearance before Congress.

There’s just one problem: “Quickly” in civil litigation translates...