Who is Jeffrey Rosen, the man set to take over the Justice Dept. for the last month of Trump’s presidency?

Since the Senate confirmed him to be President Trump’s deputy attorney general in 2019, Jeffrey A. Rosen has kept a low profile.

But with Attorney General William P. Barr’s pending resignation, Mr. Rosen is set to be the nation’s top law enforcement official for the delicate final month of Mr. Trump’s presidency. It will be an extraordinary responsibility for a man who has no prosecutorial experience — and who has participated in several decisions in which the department took steps that favored the president’s friends or punished his perceived enemies.

When Mr. Trump took office, he appointed Mr. Rosen as the No. 2 official at the Transportation Department. There, he pushed to roll back tailpipe emissions rules to let the automotive industry manufacture cars that will put more greenhouse gas pollution into the atmosphere. In 2019, when Mr. Barr became attorney general, he asked Mr. Trump to nominate Mr. Rosen as his deputy.

Mr. Rosen has had a largely quiet tenure, staying in the shadow of Mr. Barr, who has taken personal responsibility for politically tinged interventions, like reducing a recommended prison sentence for Roger J. Stone Jr., a presidential ally, and trying to drop a case against another, Michael T. Flynn, even though he had pleaded guilty. (Mr. Trump ultimately commuted Mr. Stone’s sentence and pardoned Mr. Flynn.)

Mr. Barr’s record of politicizing the Justice Department, however, has been complicated by his approach to the election. He publicly rejected Mr. Trump’s spurious claims of election fraud and kept the department out of the dozens of failed lawsuits the Trump campaign and its allies have brought seeking to subvert the outcome.

It will now fall to Mr. Rosen to decide whether to maintain Mr. Barr’s stances on such issues during what may be a contentious endgame for Mr. Trump.

Samuel Buell, a former federal prosecutor who teaches at Duke University School of Law, noted that if Mr. Rosen, during his brief tenure as attorney general, were to issue any “off the wall” edicts at Mr. Trump’s request, career department employees could try to slow them bureaucratically.

“A sustained effort to steer a department in a particular direction takes more than a month,” Mr. Buell said.