WASHINGTON (Reuters) - U.S. Transportation Department Deputy Secretary Jeffrey Rosen is the leading candidate to replace Rod Rosenstein as deputy attorney general to serve alongside new Attorney General William Barr, two administration officials told Reuters on Friday.

Barr, who previously worked with Rosen at the law firm Kirkland & Ellis, has advocated for Rosen to get the No. 2 top slot at the Justice Department, the sources said, speaking anonymously because the announcement has not been made public.

Rosen, who has been a key figure in efforts to rewrite fuel efficiency regulations and set drone policy, did not return messages seeking comment.

A spokeswoman for the Justice Department declined to comment, while the White House did not immediately respond to requests for comment.

Reuters could not immediately learn when Rosenstein would depart his post.

However, Reuters previously reported that Rosenstein planned to exit sometime after Barr was sworn in, but would initially remain on the job for an unspecified time to help with the transition before departing.

If the White House proceeds with nominating Rosen as planned, he will still need to win approval from the U.S. Senate.

The deputy attorney general role generally does not garner the spotlight as much as the attorney general, but Rosenstein rose to become a nationally-recognized figure for his decision to appoint Special Counsel Robert Mueller to oversee a probe into whether President Donald Trump’s campaign colluded with Russia.

Trump has denied colluding with Russia, and called Mueller’s probe a witch hunt.

The task of overseeing Mueller’s still-ongoing investigation will now fall to Barr, who was confirmed in a 54-45 vote by the Senate on Thursday and had a jam-packed first full day at the Justice Department on Friday which included a lunch with FBI Director Chris Wray, a meeting with senior Justice Department leaders and a nationwide call with U.S. Attorneys, a Justice Department spokeswoman said.

Barr’s new chief of staff is Brian Rabbitt, who recently served as a senior policy adviser to Securities and Exchange Commission Chairman Jay Clayton and also previously worked as a special assistant in the White House, the spokeswoman confirmed.

As for former Acting Attorney General Matthew Whitaker, he is remaining at the Justice Department for the time being where he is serving as a senior counselor in the office of the Associate Attorney General, the No. 3 post in the Justice Department that is currently being filled by Jesse Panuccio after Rachel Brand departed last year.

The office of the Associate Attorney General generally helps oversee policies and programs related to civil litigation, civil rights and antitrust.

Although Rosen will not be the main person directly overseeing the Mueller probe the way Rosenstein did, he is likely to play an important role in it.

That’s because the Justice Department’s national security division and law enforcement components report directly to the Deputy Attorney General’s office.

Rosen does not have previous experience as a prosecutor or Justice Department official, which is unusual for a deputy attorney general.

He served as general counsel at the Transportation Department from 2003 through 2006 and was later general counsel at the Office of Management and Budget from 2006 to 2009.

In 2017, Republican Senator Rob Portman, who was Rosen’s boss at OMB, said Rosen is “also one of the most respected lawyers in town” who had experience with jury trials, bench trials, contracts cases, securities cases and class actions, among other things.

“He was always really well prepared and insightful and a straight shooter,” Portman added.