The departure creates uncertainty about the oversight of Mueller's team as it enters what may be its final months of work. But the attorney general nominee, William Barr, moved quickly Wednesday to quell concerns that his arrival could endanger the probe, telling lawmakers during Capitol Hill visits ahead of his confirmation hearing that he has a high opinion of Mueller.
"He had absolutely no indication he was going to tell Bob Mueller what to do or how to do it," said Republican Sen. Lindsey Graham, the incoming chairman of the Senate Judiciary Committee, which will question Barr next Tuesday.
If confirmed by the Republican-led Senate, Barr could be in place at the Justice Department by February. Rosenstein is expected to leave his position soon after that, though he is not being forced out, said a person familiar with the plans who was not authorized to discuss them on the record and spoke on condition of anonymity to The Associated Press.
The departure is not surprising given that Rosenstein has been deputy for almost two years. It is common for new attorneys general to have their own deputies and Barr has told people close to him that he wanted his own No. 2 as part of taking the attorney general job.
It was unclear who might replace Rosenstein, though Barr has some ideas for a selection, Graham said, without elaborating. The deputy position requires Senate confirmation. It was also not immediately clear whether Rosenstein's top deputy, Edward O'Callaghan, who has a prominent role overseeing Mueller's investigation, might remain in his role.
Rosenstein's departure is noteworthy given his appointment of Mueller and close supervision of his work. He's also endured a tenuous relationship with Trump, who has repeatedly decried Rosenstein's decision to appoint Mueller, and with congressional Republicans who accused him of withholding documents from them and not investigating aggressively enough what they contend was political bias within the FBI.
Trump also shared a photo on Twitter in November showing Rosenstein and others criticized by the president behind bars, calling for them to be tried for "treason."
Democrats have been wary of Whitaker, who declined to recuse himself from overseeing the investigation. The Democratic leader of the House Judiciary Committee, Rep. Gerrold Nadler, on Wednesday called for him to appear before the panel on Jan. 29.
The White House cast Rosenstein's departure as his choice. Spokeswoman Sarah Huckabee Sanders said Rosenstein had always planned to stay two years and wants to help with the transition to a new attorney general. The person familiar with Rosenstein's plans said Rosenstein told Barr in a private conversation around the time Barr was selected that he expected to depart after Barr was confirmed.
"I don't think there's any willingness by the president or the White House to push him out," Sanders told Fox News.
Rosenstein appointed Mueller as special counsel in May 2017 to investigate potential coordination between Russia and the Trump campaign to sway the 2016 election. The appointment followed the recusal of Sessions because of his work on the Trump campaign and Trump's firing of former FBI Director James Comey.
The transition to Barr, who served as attorney general in the 1990s, would come with critical steps in the Mueller investigation expected to unfold in the weeks and months ahead, when the special counsel's office is expected to report its findings to the Justice Department.
Barr has criticized the Mueller investigation, including an unsolicited memo he sent the Justice Department last year critiquing Mueller's investigation into whether the president had sought to obstruct justice by firing Comey.
Rosenstein in December said Mueller's investigation would be "handled appropriately" no matter who is overseeing it. He said Barr would be an "excellent attorney general when he is confirmed."
Graham suggested after his meeting with Barr that there were no reasons to worry that he'd undermine the investigation.
He noted that the two men worked together when Barr was attorney general between 1991 and 1993 and Mueller oversaw the department's criminal division. Graham said the two men were "best friends," that their wives attended Bible study together and that Mueller had attended the weddings of Barr's children.
Graham listed a number of questions that he had put to Barr: "I asked Mr. Barr directly, 'Do you think Mr. Mueller is on a witch hunt?' He said no. 'Do you think he would be fair to the president and the country as a whole?' He said yes. 'And do you see any reason for Mr. Mueller's investigation to be stopped?' He said no. 'Do you see any reason for a termination based on cause?' He said no. 'Are you committed to making sure Mr. Mueller can finish his job?' Yes."
Associated Press writers Mary Clare Jalonick and Darlene Superville contributed to this report.