(CNN) -- The U.S. Sentencing Commission voted Thursday to reduce sentencing guidelines that could shorten prison stays for about 70% of federal drug trafficking defendants, backing a proposal supported by the Justice Department and some conservative lawmakers.
The unanimous vote by the commission sets in motion a change in the formula used to determine sentences for federal drug offenders. If Congress doesn't object to the change, it would go into effect on November 1.
"This modest reduction in drug penalties is an important step toward reducing the problem of prison overcrowding at the federal level in a proportionate and fair manner," said Judge Patti B. Saris, chair of the commission. "Reducing the federal prison population has become urgent, with that population almost three times where it was in 1991."
Attorney General Eric Holder testified in support of the reduced sentencing guidelines last month and he hailed the vote Thursday.
He said it "represents a milestone in our effort to reshape the criminal justice system's approach to dealing with drug offenses. This reduction in the federal sentencing guidelines, while modest, sends a strong message about the need to reserve the harshest penalties for the most serious crimes."
Holder months earlier instructed prosecutors to start using their discretion to help accomplish some of the same goals, a move that some lawmakers and at least one federal judge on the sentencing commission interpreted as bypassing the legal authority of Congress and the commission.
Judge William H. Pryor Jr., who serves on the U.S. Court of Appeals for the 11th Circuit and is a member of the sentencing commission, said he wasn't pleased that "before we voted on the amendment, the attorney general instructed assistant United States attorneys across the nation not to object to defense requests to apply the proposed amendment in sentencing proceedings going forward. That unprecedented instruction disrespected our statutory role."
The federal prison system holds 216,000 prisoners. With only 5% of the world's population, the Unites States has nearly 25% of its prisoners.
Holder, in a statement Thursday, called on Congress to make further changes to sentencing guidelines, particularly those that reduce the discretion of judges on how to deal with drug offenders.
He said legislation he is championing with some Republican lawmakers, such as Sen. Rand Paul, and Democrats, such as Sen. Patrick Leahy, "would take further steps to reduce our overburdened prison system."