Meanwhile on Wednesday, the Senate Judiciary Committee held a confirmation hearing for Jeffrey A. Rosen, whom Mr. Trump has nominated to be Mr. Barr’s deputy. Mr. Rosen, who is currently the deputy secretary of transportation, would succeed Rod J. Rosenstein, the deputy attorney general who appointed Mr. Mueller.

Mr. Rosen largely sidestepped questions about the Russia investigation and Mueller report, saying he was not part of the department yet and did not know the facts. But Senator Amy Klobuchar, Democrat of Minnesota, noted that some criminal matters that developed from Mr. Mueller’s work are continuing even though the Office of the Special Counsel is shutting down, and she pressed Mr. Rosen to commit to letting them proceed without political interference.

“If I am confirmed,” Mr. Rosen replied, “I would expect in all prosecutorial matters to proceed on the facts and the law and not any improper political influences.”

Ms. Klobuchar did not name which matters she had in mind, though several cases are still working their way through the court system, including the indictment of Roger J. Stone Jr., a longtime informal adviser to Mr. Trump.

Separately, under questioning by Senator Dianne Feinstein, Democrat of California, Mr. Rosen defended his role in creating a Trump administration proposal on tailpipe pollution rules. Ms. Feinstein referred to a New York Times article that portrayed him as pushing to let cars emit more greenhouse gases. Mr. Rosen said the article had several errors, although he did not identify them.

He also said the issue was how to carry out a law that required annual increases in fuel economy standards through 2020, and then set criteria for decisions about whether to require any further escalations in the years after that. The Trump administration, he said, was merely proposing a “timeout” after 2020.

In fact, the current regulations, put forth by the Obama administration, require automakers to keep improving the fuel economy of passenger vehicles through 2025. The administration’s preferred proposal would therefore allow more greenhouse gas pollution than current rules.