But after Congress passed the law, they expressed concern that Justice Department officials who had sided with Mr. Sessions and remained in their jobs after he was fired in November would delay its implementation.

“Sessions represented an all-time low for prospects for successful passage at the time and also prospects for implementation,” said Holly Harris, the president of Justice Action Network, a criminal justice reform advocacy group. “We’re looking at a vastly improved situation,” she said of Attorney General William P. Barr, who replaced Mr. Sessions in February.

Mr. Barr assured supporters of the law that he would back its full implementation. Ahead of his confirmation hearing, he met with senators including Charles E. Grassley, Republican of Iowa and the former head of the Senate Judiciary Committee, who was one of the bill’s strongest backers. Mr. Barr said that his thinking on criminal justice had evolved, and that he no longer supported the harsh sentencing measures that he had pushed for during his first stint as attorney general in the early 1990s.

During an interview this month after touring a federal prison in Edgefield, S.C., Mr. Barr defended the tough-on-crime measures he had supported at the time as “essential for reducing the crime rate and also tackling the crack epidemic.” But with crime rates down substantially, he said he felt that the time was right to identify and free inmates who no longer constituted a safety risk.

“After someone has been in prison for a substantial period of time, and you can really assess whether they continue to pose a threat to the community,” Mr. Barr said, “then obviously you’re more inclined to modify the sentence or strike the balance in favor of some kind of monitoring that doesn’t involve the heavy cost and the isolation of this kind of prison system.”

The law consists of a package of incentives and new programs designed to improve prison conditions and prepare prisoners considered low risks for recidivism to re-enter society. It also outlawed the shackling of pregnant inmates and the placement of juveniles in solitary confinement and said the Bureau of Prisons must place prisoners close to home if possible.

The law also makes retroactive the 2010 Fair Sentencing Act, which decreased the relative penalty for possession of crack versus powder cocaine.