Attorney General Eric H. Holder's plan to reduce overcrowding in federal prisons by instructing federal prosecutors to stop invoking mandatory minimum sentences against low-level, nonviolent drug offenders was a welcome, if overdue, announcement. The policy's chief shortcoming is that Mr. Holder can't, by himself, correct this long-standing problem. That will require intervention by Congress — as well as by legislatures in states where similar problems exist.

That the United States locks up too many people for too long is unquestionably true. It doesn't take a card-carrying member of the American Civil Liberties Union to recognize this either. A growing number of state houses have already been taking up mandatory minimum sentences because incarcerating so many people has gotten outrageously expensive. In Maryland, the price for housing and feeding 22,000 inmates exceeds three-quarters of a billion dollars annually.

The numbers speak for themselves. While the nation's overall population has grown by about one-third since 1980, its prison population has swelled by nearly eight-fold. The U.S. may represent just 5 percent of the world's population, but it accounts for more than a quarter of all prisoners. Many of these are small-time actors in the well-intentioned but often misguided "war on drugs." As a result, federal prisons are now filled 40 percent beyond capacity.

Making matters worse, this so-called justice is clearly meted out unequally in regard to race and class. As a study released earlier this year pointed out, black men have been given sentences 20 percent longer than white males convicted of similar crimes. Mr. Holder calls this "shameful," and he is correct.

What the attorney general chose to do about this comes under the mantle of prosecutorial discretion. U.S. attorneys will now, for instance, pick and choose whether to indicate the quantity of drugs involved with certain defendants on charging documents. Specifying certain amounts can invoke harsh mandatory minimum sentences. Judges will still have the discretion to dole out these tougher sentences, but it will no longer be automatic.

Some may naively suggest that prosecutors should not have such decision-making power, but that has always been part of the judicial process. Just as one of Mr. Holder's predecessors, John Ashcroft, had the authority to tell his attorneys to always charge the "most serious, readily provable offense" 10 years ago, Mr. Holder can choose otherwise. Prosecutors have always had the authority to choose which charges to pursue.

It would be even better for Congress to take up the issue. State-level reforms have been approved in recent years by Democrats and Republicans alike from Ohio to Texas. But it's safe to say that things have become so dysfunctional on Capitol Hill that serious reform — even the pending legislation with sponsors from both political parties — is a long-shot at best.

Nevertheless, ending the costly and counter-productive policies that have lead to the current mass incarceration ought to be a high priority in Washington and elsewhere. Nothing Mr. Holder can do now can address current inmates, including the elderly whose continued incarceration serves no earthly purpose, or those facing mandatory sentences in state prisons, which house more than 80 percent of all inmates.

But it is a step in the right direction and should encourage lawmakers to follow suit. Why didn't Mr. Holder take this action sooner? Unfortunately, we suspect it was the same sort of politics that have hampered reforms in the past — a fear that advocating any rollback of mandatory minimum sentences will be perceived as being "soft" on crime. Considering how much more effective it would be to spend those billions of dollars now spent on prisons ($80 billion annually to be precise) on programs that actually reduce crime, such reforms aren't soft at all.

It is encouraging to note, too, that the attorney general's announcement coincides with a federal judge's ruling that New York City's stop-and-frisk program amounted to a form of racial profiling and thus violated the Constitution. Programs that target blacks and other minority groups are another reason why prison sentences are meted out so unfairly. Tough-on-crime laws are nothing of the sort if they merely result in putting more minorities in prison for longer periods of time.