Zebley will be there to advise Müller, but the special lawyer will be the only one to answer the committee questions, the source said. Democrats only plan to swear Müller as witnesses.
Müller requested at the last minute that his deputy be admitted to the Miller Report for Wednesday's Blockbuster hearings before the Judiciary and Intelligence Committee if he had to help with any questions that the Special Adviser, according to a source familiar with the affair, did not could completely answer.
Republicans in the House Judiciary Committee immediately expressed concern that the Democrats made a last-minute change to the hearing to testify to Zebley, in addition to the Special Representative, less than 24 hours before the hearings began.
The prospect of Mueller's deputy standing next to the Special Representative is a last-minute turn in one of the most anticipated hearings in years. Even if Zebley were only there as a lawyer for Müller, this could change the dynamics of Müller's statement on Wednesday before the committees.
The Department of Justice denied Zebley or other persons who testified alongside Mueller because they were not authorized to a person who is familiar with the matter. The practice of the department generally excludes the statement of line attorneys.
But the bottom line is that Zebley and many others who were in Miller's staff are now private individuals and the Justice Department can only object.
A spokesman for Mueller did not respond to comments.
Democrats have urged Zebley, along with another Mueller member, James Quarles, to hear behind closed doors after Mueller testified. However, the Department of Justice also opposed this request, and Attorney General William Barr said he would decline a summons if Congress would enforce her testimony.
The committee's top republican, Doug Collins of Georgia, accused the Democrats of "surprising a witness" and said he was able to testify to Zebley would be an "unprecedented decision to allow a witness's attorney to both privately "If Democrats Believe, It's the Attorney's Responsibility to Give Testimony Report that you have no reason to outsource this task at the expense of the integrity of our committee," said Collins the explanation.
Zebley was a long-time adjunct to Muller's side and served as Mueller's chief of staff when he was FBI director. He also worked with Mueller in the private law firm Wilmer Hale before joining his team after Mueller was appointed Special Adviser.
This story was updated on Tuesday with additional developments.