Former special counsel Robert Mueller will have a sidekick Wednesday when he testifies to House lawmakers.

Aaron Zebley, Mr. Mueller’s former chief of staff and his right-hand man during the Russia investigation, will appear alongside his former boss to help with testimony, a source familiar with the process confirmed to The Washington Times.

Although Mr. Zebley can advise his former boss, he will not be answering questions, the source said.

Mr. Mueller is slated to appear before the Judiciary Committee in the morning and the intelligence committee in the afternoon. Mr. Zebley will be sworn in for the earlier proceeding, but it’s not clear if he’ll be part of the intelligence panel’s testimony.

Jim Popkin, a spokesman for Mr. Mueller, said the arrangement was discussed with the committees more than a week ago.

“Aaron Zebley was the deputy special counsel and had day-to-day oversight of the investigations conducted by the office,” Mr. Popkin said in a statement.

It is not clear why Mr. Mueller requested Mr. Zebley’s assistance, and the move left Republicans wondering about motives.

Rep. Doug Collins, the top Republican on the Judiciary Committee, worried it was the Democrats’ attempt to circumvent the Justice Department’s demand that Mr. Mueller stay within the bounds of his 448-page report. No longer a Justice Department employee, Mr. Zebley would not be bound by the guidance.

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

Later on Tuesday evening, Rep. Adam Schiff, California Democrat and chairman of the intelligence committee, explicitly asked Mr. Mueller to ignore Justice Department efforts to limit his testimony to his already-released report.

“The Committee likewise expects that the untenable position the Department has staked out with regard to executive privilege will have no impact on your testimony,” he wrote.

While it is not unusual for witnesses to have assistance from counsel or others, they typically sit behind the witness, not alongside them. Republicans accused Democrats of changing the rules less than 24 hours before the blockbuster hearing.

“You don’t get to change the rules right before kickoff, especially after a 22-month, $30 million investigation,” Rep. Jim Jordan, Ohio Republican, said on Twitter.

Mr. Collins said the change, which he called a “stunt,” may jeopardize whether the hearing complies with the rules of the House.

President Trump took to Twitter on Tuesday evening to express his dissatisfaction, calling it further proof of “a rigged Witch Hunt!”

“Just got back only to hear of a last minute change allowing a Never Trumper attorney to help Robert Mueller with his testimony before Congress tomorrow. What a disgrace to our system. Never heard of this before,” he wrote.

Spokespeople for House Judiciary Chairman Rep. Jerrold Nadler and the Justice Department did not respond to requests for comment.

Mr. Schiff said Tuesday that Mr. Mueller had requested Mr. Zebley’s appearance before his panel, too. He said those discussions were continuing. Mr. Schiff said he had concerns about expanding the witness table, fearing the session would be “converted to a hearing with someone else.”

Mr. Schiff was already seeking closed-door testimony from Mr. Zebley and another Mueller deputy, James Quarles. It is not known if those appearances have been scheduled.

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