WASHINGTON — Aaron Zebley, a top prosecutor in the special counsel’s office, is so close with his former boss, Robert S. Mueller III, that he wears the same kinds of pinstripe suits, white dress shirts and Casio watch.

On Wednesday, Mr. Zebley sat at the witness table with Mr. Mueller during two highly anticipated hearings on the Russia investigation, an unusual arrangement that House Democrats signed off on Tuesday after Mr. Mueller made a last-minute request. Mr. Zebley, 49, was to help Mr. Mueller answer questions about the sprawling investigation and the 448-page report documenting it, a committee official familiar with the hearing said.

[Mr. Mueller testifies before Congress.]

The news prompted President Trump to post a series of accusations about Mr. Zebley, including that he is a “Never Trumper” lawyer, a reference to prosecutors on Mr. Mueller’s team he believes were opposed to him. There is no evidence of that claim. Mr. Zebley is not politically affiliated and according to a Washington Post review, has not donated to candidates from either party.

Republicans on the House Judiciary Committee also criticized the request as a circumvention of House rules. Representative Doug Collins of Georgia, the top Republican on the panel, said Democrats were “outsourcing” Mr. Mueller’s responsibility to testify.