Attorney General Jeff Sessions at the Justice Department. (Michael Reynolds/European Pressphoto Agency)

ATTORNEY GENERAL Jeff Sessions announced on Friday a new Justice Department policy designed to “enforce the law fairly and consistently.” It will not achieve that aim. Instead, the attorney general’s plan unintentionally underscores the case for Congress to reform the nation’s unjust sentencing laws.

Mr. Sessions withdrew a sentencing policy crafted by Obama Attorney General Eric H. Holder Jr., which has been in effect since 2013. In its place, Mr. Sessions commanded federal prosecutors to “charge and pursue the most serious, readily provable offense,” a policy that will undoubtedly put some people in prison longer than is just or practical.

Mr. Holder’s policy had effectively softened harsh mandatory sentences for lower-level offenders. Even the body in charge of sentencing guidelines, the federal Sentencing Commission, concluded several years ago that sentencing standards had cracked down too hard on relatively minor offenders, such as drug couriers and mules, rather than being focused on kingpins and other major players. But under the law, the commission was unable to fully address the problems it identified. Mr. Holder also faced legal constraints, though he pressed against them aggressively: His policy involved declining to list the quantity of drugs in the possession of certain petty, nonviolent defendants, so as not to trigger automatic, harsh sentences. In a response to Mr. Sessions’s move obtained by MSNBC’s Rachel Maddow, Mr. Holder noted on Friday that federal prosecutors focused on “more serious drug cases” after his policy took effect.

Mr. Sessions justified his policy shift in part by arguing it would promote consistency in federal sentencing. But some prosecutors still will shrink from throwing the book at defendants, while others will not. Moreover, if every new attorney general dramatically shifts charging policy, over time a great many people who committed the same crime may be locked up for very different periods.

Mr. Sessions’s new policy is not maximally draconian. At a Friday news conference, the attorney general noted that he will allow prosecutors “to avoid sentences that would result in an injustice.” But getting an exception from his charge-the-maximum policy will require supervisor authorization and a written explanation. That will likely happen much less often than it should.

Mr. Holder’s policy was not perfect either. It seemed to order prosecutors to change the crime to fit the punishment, in tension with what the law demanded. The right response to this imperfection, though, is not to return to a differently flawed sentencing policy, but for Congress to act. Current law still reflects the get-tough-on-crime mood of decades ago that a growing, bipartisan group of officials has concluded is outmoded, expensive and, often, counterproductive.

Congress should not have dithered while states and the executive branch attempted to ease the sting of the system it created. Now, as the attorney general reimposes the full severity of the laws Congress passed, the need to move is only greater. Lawmakers established mandatory minimum sentences. Lawmakers ultimately must be the ones to reshape them.