Attorney General Eric H. Holder Jr. is endorsing a proposal that would reduce prison sentences for people convicted of dealing drugs, the latest sign of the Obama administration’s retrenchment in the so-called war on drugs.

In January, the United States Sentencing Commission proposed changing federal guidelines to lessen the average sentence for drug dealers by about one year, to 51 months from 62 months. Mr. Holder testified before the commission on Thursday in support of the plan.

With the support of several Republicans in Congress, the attorney general is separately pushing for the elimination of mandatory minimum sentences for nonviolent drug crimes. In January, the Justice Department issued a call encouraging low-level criminals serving lengthy sentences on crack cocaine charges to apply for clemency.

Since the late 1970s, the prison population in the United States has ballooned into the world’s largest. About one in every 100 adults is locked up.