WASHINGTON — Rod Rosenstein was "one of the Justice Department's most important leaders in its history," former Attorney General Jeff Sessions said Thursday at a farewell ceremony for Rosenstein.

Rosenstein, who as deputy attorney general supervised special counsel Robert Mueller's investigation, steps down on Saturday. President Donald Trump has nominated Jeffrey Rosen, the deputy secretary of the Department of Transportation, to be his successor. The full Senate could vote on the nomination as early as Monday.

On Thursday, Rosenstein was praised by Sessions, Attorney General William Barr, FBI Director Chris Wray and other officials during a ceremony in the Justice Department's ornate Great Hall.

Barr was deputy attorney general when Rosenstein first arrived at the Justice Department in 1990.

"In those days, the deputy job was a lot different. But I'll tell you now, the attorney general job is a lot different also. This must be a record, of an attorney general being proposed for contempt within 100 days of taking office," Barr said with a smile.

Reviewing Rosenstein's record, Barr said: "As a law student at Harvard, Rod interned in the U.S. Attorney's office in the district of Massachusetts, and the acting U.S. Attorney in that office at the time was none other than Bob Mueller. Bob went on to be assistant attorney general for the Criminal Division where Rod was first assigned as an honors program lawyer. Little did we know that we'd be getting the old band back together again."

Sessions noted that Rosenstein served during an especially turbulent time.

"When we came in, I had no doubt there would be a lot of controversies during my tenure," Sessions said. "The office of attorney general frequently finds itself at the vortex of law and politics and separation of powers. But in truth, I have to say, our run exceeded my expectations."

It was Rosenstein who called for a special counsel and appointed Mueller to lead the investigation, after Sessions recused himself from the Russian meddling investigation because of his role in the Trump campaign.

"He stayed the course during some of the most difficult times in the history of the department," Sessions said.