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(CNN) —  

Special counsel Robert Mueller’s deputy Aaron Zebley is expected to appear next to the special counsel as his counsel at Wednesday’s hearings on the special counsel’s report, according to a House Judiciary committee source.

Zebley will be there to advise Mueller, but the special counsel will be the only one answering the committee questions, the source said. Democrats on the House Judiciary Committee only plan to swear in Mueller as a witness. However, Zebley will be sworn in to testify in front of the House Intelligence Committee, a committee aide told CNN.

Mueller made the last-minute request to have his deputy sworn in for Wednesday’s blockbuster hearings before the Judiciary and Intelligence Committees on the Mueller report, in case he needed to help with any questions the special counsel could not fully answer himself, according to a source familiar with the matter.

“Aaron Zebley was the Deputy Special Counsel and had day-to-day oversight of the investigations conducted by the Office. He will accompany Special Counsel Mueller to the Wednesday hearings, as was discussed with the committees more than a week ago,” said Mueller spokesman Jim Popkin.

Republicans on the House Judiciary committee immediately raised concerns that Democrats were making a major, last-minute change to the hearing to have Zebley to testify alongside the special counsel, less 24 hours before the hearings are scheduled to begin.

The prospect that Mueller’s deputy could appear alongside the special counsel is a last-minute twist in one of the most highly anticipated hearings in years. Even if Zebley was there just as a counsel to aid Mueller, it could still change the dynamic of Mueller’s testimony Wednesday before the committees.

President Donald Trump weighed in on Twitter Tuesday evening, calling Zebley’s inclusion a “disgrace to our system.”

“VERY UNFAIR, SHOULD NOT BE ALLOWED,” he tweeted.

The Justice Department objected to Zebley or any others testifying alongside Mueller because it wasn’t authorized, according to a person familiar with the matter. The department’s practice generally precludes line attorneys from testifying.

But the bottom line is that Zebley and many others who were on Mueller’s staff are private citizens now, and the Justice Department cannot do much other than object.

Democrats have pushed to hear from Zebley along with another Mueller deputy, James Quarles, behind closed doors after Mueller testified. But the Department of Justice also resisted that request, and Attorney General William Barr said he would oppose a subpoena if Congress tried to compel their testimony.

The committee’s top Republican, Rep. Doug Collins of Georgia, accused Democrats of adding “a surprise witness” and said that allowing Zebley to testify would be a “unprecedented decision to allow a witness’s counsel to both advise him privately and simultaneously testify alongside him.”

“If Democrats believe it is the special counsel’s responsibility to testify to his report, they have no ground for outsourcing that duty at the expense of our committee’s integrity,” Collins said in the statement.

Later Tuesday, Collins said he had spoken to House Judiciary Chairman Jerry Nadler, who assured him that Zebley would only serve as counsel and would not speak at the hearing. He did not take issue with that arrangement, he said.

“The chairman is not going to allow him to speak,” Collins told reporters.

Zebley has long been an aide by Mueller’s side, serving Mueller’s chief of staff when he was FBI director. He also worked with Mueller at the private law firm Wilmer Hale before joining his team after Mueller was appointed special counsel.

Lawmakers from both parties went through their final preparations ahead of Wednesday’s hearing, with Intelligence and Judiciary lawmakers huddling in closed-door sessions.

Norm Eisen, a counsel on the House Judiciary Committee, played Mueller during a mock hearing with Democrats on the committee, according to multiple sources. Judiciary Republicans and Intelligence Democrats have also held mock questioning sessions with aides taking on the special counsel role.

At the Judiciary Democrats’ mock hearing Tuesday, another aide also played the role of Republican Rep. Jim Jordan of Ohio. A source says the reason Democrats felt the need to prep for Jordan is over concerns that GOP members are going to interrupt the hearing to try to kill Democratic momentum.

Nadler told reporters after the session, “I don’t know,” when asked if Mueller would change the course on impeachment.

“My hope for tomorrow … is the Mueller investigation revealed a lot of conduct about the President, which the American people should be aware of,” Nadler said. “The President and the attorney general have systematically lied to the American people about what was in that report. … They have said no obstruction, collision and he was totally exonerated – all three of those statements were not true.”

This story has been updated with additional developments Tuesday.