Former special counsel Robert S. Mueller III’s longtime aide will appear beside him during a much-anticipated hearing Wednesday, and be sworn in for testimony at another, a last-minute request that caused a stir just hours before the high-profile congressional event.

Democrats on the House Judiciary Committee agreed to allow Aaron Zebley, a former top deputy to Mueller, to sit beside him and advise him in his answers. The accommodation came after Mueller asked that Zebley be allowed to testify as a witness next to him, a request Judiciary Democrats rejected. The House Intelligence Committee, meanwhile, has agreed to let Zebley be sworn in for its hearing later in the day, according to a committee aide.

Zebley was Mueller’s chief of staff when Mueller was FBI director, and he played largely the same role inside the special counsel’s office. He and another top deputy, James Quarles, had been helping Mueller prepare for his testimony in unused office space at the WilmerHale law firm, which Mueller left to lead the special counsel investigation.

“The special counsel’s office has made it clear for some time, they’d like to have him present, they would like to allow him to answer questions,” said House Intelligence Committee Chairman Adam B. Schiff (D-Calif.). “We want this hearing to be about Director Mueller.” 

Reacting to the news, President Trump tweeted Tuesday: “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. VERY UNFAIR, SHOULD NOT BE ALLOWED. A rigged Witch Hunt!”

There is no evidence Zebley is a “Never Trumper.” While many on Mueller’s team gave political donations to Democratic candidates, Zebley has no political donation history, records show.

The request infused an air of uncertainty into last-minute planning as lawmakers worried about whether Zebley would steal a spotlight they hope to keep on Mueller himself. The Justice Department also opposed the idea of allowing Zebley to give sworn testimony.

Jim Popkin, a spokesman for Mueller, said in a statement: “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.”

One person familiar with the matter said that Mueller made the request to have Zebley be sworn in and testify in the last 24 hours or so. The Justice Department did not authorize Zebley to testify and objects to his doing so, said the individual, who spoke on the condition of anonymity to share private deliberations.

Republicans decried the move as an 11th-hour trick, arguing Democrats would have had to announce another witness days ago under traditional procedures in the House. In a statement Tuesday afternoon, Rep. Douglas A. Collins (Ga.), the top Republican on the Judiciary Committee, accused the chairman, Rep. Jerrold Nadler (D-N.Y.), of “again allowing the committee’s business to devolve into chaos.”

“If true, the chairman’s unprecedented decision to allow a witness’s counsel to both advise him privately and simultaneously testify alongside him shows the lengths Democrats will go to protect a one-sided narrative from a thorough examination by committee Republicans,” Collins said. “ . . . A last-minute addition to the witness list would now jeopardize whether tomorrow’s hearing complies with the rules of the House.”

Mueller is scheduled to testify for three hours before the Judiciary Committee beginning at 8:30 a.m. and then answer questions from the House Intelligence Committee for two hours.

The committees have been seeking his testimony on the nearly two-year investigation into Russian interference in the 2016 election and whether President Trump tried to obstruct justice.