The effectiveness of an investigation into alleged misconduct by the California Supreme Court was thrown into question today when one of the justices, Stanley Mosk, filed a court petition charging the public proceding was unconstitutional.

In his petition in state Superior Court, Mosk sought to quash a subpoena from the Commission on Judicial Performance requiring him to testify in this inquiry into whether the court delayed release of controversial decisions for political reason. Mosk claimed that a clause in the state constitution requires the proceedings and investigations to be carried on in private.

While a ruling in favor of Mosk will not directly affect the rest of the commission's investigation, it would open the door to similar constitutional objections by other witnesses. At least one other justice, William Clark, has indicated he might challenge the public proceedings.

The loss of Mosk's testimony alone, however, could crippled the commission's effort to determine if there was any wrongdoing by the court. Several discrepancies that have surfaced in the three-week-long hearing could be cleared up by Mosk's testimony.

Most crucial in this regard would be resolution of a dispute over whether Mosk told another justice, Mathew Tobriner, last fall that he felt the court was delaying release of several decisions until after the November 7 confirmation election of three of the justices.

Tobriner has testified he in effect had no such conversation with any justice at the time. But Clark stated that Mosk twice told him of his confrontation with Tobriner.

The commission's special counsel, Seth Hufstedler, agreed that Mosk's testimony was "important" to several aspects of the investigation. He said, however, he "expected" the Superior Court would rule against Mosk.

The issue will be heard Wednesday and a ruling is expected soon afterward.