His absence has left a disappointed House majority with little option but to stage political theatrics like Thursday’s 12-hour reading of the entire redacted Mueller report in a hideaway Capitol committee room before a few reporters, a guest appearance by the liberal activist actor John Cusack and the unblinking eye of a C-Span camera.

“We cannot count on anyone but Mueller to tell us what he was thinking, and it should not be filtered through anyone else — seeing is believing, hearing is believing,” said Representative Adam B. Schiff, Democrat of California and the chairman of the House Intelligence Committee, adding that a “sterile report” was no substitute for either Mr. Mueller or Mr. McGahn…

Democrats involved in the investigations insist that they still have options. They can hold hearings with empty chairs, summon friendly witnesses and mount new and novel challenges against the administration on health care and other issues. On myriad looming legal fights, they are confident they will ultimately prevail in court — and there is always the possibility that a high-impact witness will be willing to buck Mr. Trump and emerge like John Dean unexpectedly turned on President Richard Nixon.

But those options lack the possible impact of Mr. Mueller, the most recognizable figure behind the investigation.