“Brian is an outstanding lawyer with a diverse public service and criminal law background spanning over 20 years,” Attorney General Jeff Sessions said in a statement. “At a time like this — with surging violent crime and an unprecedented drug epidemic — this position is especially important.”

Mr. Benczkowski, 48, has worked since 2010 as a lawyer focused on white-collar criminal defense cases at the firm Kirkland & Ellis. In that job, he helped Russia’s Alfa Bank investigate whether its computer servers had contacted the Trump Organization, a question that touched directly on suspicions about the bank that emerged in the early months of the Trump-Russia affair.

The F.B.I., which also investigated, found that data moving between the bank and the Trump Organization did not amount to clandestine communications, and some experts suggested that it was related to Trump hotel marketing materials.

But Democratic senators said Mr. Benczkowski’s decision to take on the Alfa Bank work last year amid heightened scrutiny over relations between Trump associates and Russia showed a lack of good judgment. Alfa Bank’s owners have ties to President Vladimir V. Putin of Russia, and Mr. Benczkowski worked closely with the Trump transition team and was once a Senate Judiciary Committee staff member when Mr. Sessions was on the committee.

“The main criticism is that Brian will be the person in the Justice Department who oversees sensitive cases, criminal trials and people who make calls on things like search warrants,” said Joyce Vance, a former United States attorney for the Northern District of Alabama. “The flip side of that and the good news is that he’ll be surrounded by career prosecutors who will know how to do all of these things, and whose advice he will have access to.”