The body set up to remove Baath party members from positions of power in Iraq has raised objections against the judge in charge of Saddam Hussein's trial.
The wrangle over the judges is damaging the trial's credibility
The Iraq Commission said Said Hameesh was a member of the former ruling party and should be barred from the case.
Mr Hameesh, who is due to chair the panel of judges when the trial resumes next week, has denied being a Baathist.
The chief judge, Rizgar Amin, quit last week, accusing the government of interfering in the case.
A permanent replacement has not been named yet but Mr Hameesh will preside over the case next week as he is the most senior remaining judge on the panel of five.
Iraq Commission chief Ali Faisal called for him to resign, saying he was the subject of a de-Baathification inquiry.
"His presence in this court violates the statutes... and he must be replaced," he said.
Ali al-Lami, the Iraq Commission's executive director, said a formal objection had been sent to the Iraqi High Tribunal.
While the judge was not immediately available for comment, chief prosecutor Jaafar al-Moussawi defended him, saying court officials would want to see evidence before withdrawing support for him.
"Hameesh denies having any relationship with the Baath party," he told Reuters news agency.
"The Commission... must provide its evidence to prove its credibility."
Hundreds of thousands of Iraqis were members of the ruling party, many of them saying it was necessary for practical reasons such as career advancement.
The latest accusation is the second serious blow to the credibility of the trial within days, say analysts, following the disappearance of its most well-known figure from the judges' bench.
Former Iraqi president Saddam Hussein and seven members of his regime are being tried over the killing of 148 people in Dujail in 1982. The defendants all deny the charges.