Ty Cobb, a special counsel to the president, declined to comment, saying only that the White House would “continue to fully cooperate” with Mr. Mueller’s inquiry. He has frequently said that the White House would cooperate with Mr. Mueller’s investigation and that he hoped it would be completed quickly. Mr. Priebus did not return messages seeking comment.
Mr. Mueller has expressed interest in speaking with other administration officials, including members of the communications team. But Mr. Trump’s allies are particularly concerned about Mr. Mueller’s interest in talking to Mr. Priebus, a former chairman of the Republican National Committee who worked closely with Mr. Trump during the presidential campaign. Mr. Trump’s confidants at the White House say Mr. Trump was never fully convinced that Mr. Priebus would be loyal to him.
Shortly after the November election, Mr. Priebus was made chief of staff, and he was involved in the major decisions the president made during the transition and in the first six months of the administration. Mr. Priebus made a point of being in most meetings and tried to be aware of what the president was doing. Mr. Trump fired him last month.
Mr. Priebus can potentially answer many questions Mr. Mueller has about what occurred during the campaign and in the White House. Mr. Priebus appears on the calendar of Mr. Manafort on the same day in June 2016 that Mr. Manafort and other campaign officials — including Mr. Trump’s eldest son and son-in-law — attended a meeting with Russians who claimed to have damaging information about Hillary Clinton, according to two people briefed on the matter. It is not clear whether Mr. Priebus and Mr. Manafort met that day.
According to a law enforcement official briefed on the investigation, Mr. Comey met with Mr. Priebus at the White House on Feb. 8 — a week before Mr. Comey said Mr. Trump cornered him in the Oval Office and asked him to end an investigation into Mr. Trump’s former national security adviser, Michael T. Flynn. In Mr. Comey’s meeting with Mr. Priebus, Mr. Comey told Mr. Priebus about a Justice Department policy that largely bars discussions between White House officials and the F.B.I. about continuing investigations in order to prevent political meddling — or at least the appearance of it — in the bureau’s work, according to the law enforcement official.
It is not clear whether Mr. Priebus ever relayed that message to the president. Mr. Trump’s Republican allies — including the House speaker, Paul D. Ryan — have said that Mr. Trump may have asked Mr. Comey to end the investigation because he was a new president who did not understand the subtleties of how the commander in chief should interact with the F.B.I.
Mr. Priebus may also be able to help prosecutors verify crucial details about Mr. Trump’s interactions with Mr. Comey. According to testimony Mr. Comey provided to Congress, Mr. Priebus knows that Mr. Comey had the one-on-one encounter with Mr. Trump on Feb. 14, when Mr. Comey has said Mr. Trump asked him to end the Flynn investigation. Mr. Trump has said that the meeting did not occur and that he did not ask Mr. Comey to end the inquiry.
Mr. Comey said in his testimony to Congress that on Feb. 14, Mr. Trump had Mr. Priebus, the attorney general, the vice president and other senior administration officials removed from the Oval Office after a counterterrorism briefing.
“The president began by saying Flynn hadn’t done anything wrong in speaking with the Russians, but he had to let him go because he had misled the vice president,” Mr. Comey said.
“The president then made a long series of comments about the problem with leaks of classified information — a concern I shared and still share. After he had spoken for a few minutes about leaks, Reince Priebus leaned in through the door by the grandfather clock and I could see a group of people waiting behind him. The president waved at him to close the door, saying he would be done shortly. The door closed.”
Right after the door closed, Mr. Comey said, Mr. Trump asked him to end the Flynn investigation.
Mr. Trump and his lawyers have tried to cast the search warrant served on Mr. Manafort as an unusual measure and an abuse of power. The president said he was surprised to learn about the search, saying it was something federal authorities “very seldom” do. John Dowd, a lawyer for Mr. Trump, said the search was similar to tactics used in Russia.
“The search warrant here was obtained by a gross abuse of the judicial process by the special counsel’s office,” Mr. Dowd told The Wall Street Journal in an email. “In addition, given the obvious unlawful deficiencies, this extraordinary invasive tool was employed for its shock value to try to intimidate Mr. Manafort.”
He added, “These methods are normally found and employed in Russia not America.”