DURING THE urban crime crisis of decades ago, federal and state governments cracked down on crime. Tough measures, such as harsh sentencing laws, led to an eight-fold increase in the federal prison population since 1980. These laws helped lower crime rates, but the increase in incarceration was expensive and imprisoned some people for longer than they deserved.

So Attorney General Eric H. Holder Jr. has been using the powers of the Justice Department to reform how the federal government punishes drug offenders. Last week he asked the arbiter of national sentencing rules, the U.S. Sentencing Commission, to chip in.

The commission had begun a broad rebalancing of the nation’s criminal justice system before Mr. Holder came calling. Its current plan is to reduce the severity of charging recommendations for a variety of lower-level drug crimes. Though tough minimum-sentencing guidelines were supposed to be aimed at bigger fish, the commission found that they often netted minnows, catching couriers and mules rather than suppliers and organizers. The commission found that the higher up offenders were in the illegal drug trade, the more likely they were to get hit with tough minimum sentences. But half of federal drug cases in 2009 were brought against people in the lower rungs, and many were still subject to minimum penalties.

The commission can do only so much by itself. So Mr. Holder is instructing prosecutors to charge low-level, nonviolent drug offenders more leniently by, for example, writing indictments in ways that don’t trigger minimum-sentencing guidelines.

Congress is best situated to reform sentencing policy; the law must change before the commission can do much more, and there are limits to the discretion Mr. Holder can or should use as chief prosecutor. Besides, another administration could easily reverse course unless lawmakers reset policy. The commission has endorsed broader legislative change.

Many prosecutors oppose leniency, arguing that tough sentencing guidelines give them more leverage. That’s true, but it’s no defense of punishments that are tougher than they should be. Some prosecutors also argue that minimum sentences promote consistency; yet the Sentencing Commission found that the reverse is often true because many prosecutors and judges weren’t comfortable throwing the book at offenders.

The critics are right about one thing: Saving money can’t be the primary reason for reform. But when fiscal sanity meshes with the goal of more fairly matching crime and punishment, officials should be thankful for a rare win-win.