If AG Barr continues to stonewall, then Nadler should bring out the big guns. He has the power to grant immunity from criminal prosecution, so he should subpoena Mueller, and grant him immunity for any Grand Jury testimony that Mueller needs to reveal in order to fully answer the questions of the Judiciary Committee. He should further subpoena and obtain under seal the names of the members of the Grand Jury, and bring them in to testify about the information they received, in order to confirm Mueller’s testimony. They should testify via video and audio from an undisclosed location, with appropriate masking of features and voice, and be identified as Grand Juror 1- Grand Juror X. Of course, Nadler would grant them immunity from prosecution as well.
I am not a lawyer, but I can’t find anything that prevents Nadler from the above, and it fully circumvents any Grand Jury secrecy laws.
I would be very interested to hear legal arguments why this strategy would be ineffective.