THE URGENCY of criminal-justice reform has become a rare matter of bipartisan consensus in Washington. Yet a reform bill introduced two years ago with support from both sides of the aisle failed to make it through the Senate. Now the bill's backers are renewing their effort to reduce the federal prison population.

The Sentencing Reform and Corrections Act, sponsored by Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) and Sen. Richard J. Durbin (D-Ill.) along with a bipartisan group of lawmakers, would be a significant step forward. But it would also be far from sufficient as a solution to overincarceration.

The bill seeks to lower mandatory-minimum sentences for nonviolent drug offenses and limit those enhanced sentences to only "serious" drug crimes — though mandatory minimums would newly apply to offenders with serious violent crimes on their records. Judges could retroactively reduce the sentences of those who received a lengthy prison term under these requirements, except for prisoners who had committed a violent felony. Likewise, the bill would allow judges to reduce harsh sentences handed down for possession of crack cocaine by retroactively applying the Fair Sentencing Act, which reduced the disparity between criminal penalties for crack and powder cocaine. And courts would have flexibility to impose sentences lower than the minimum requirement in an increased number of drug cases.

Beyond decreasing sentences, the bill puts in place measures to reduce recidivism and allow a smoother reentry into society for offenders who have served their sentences. Certain prisoners with nonviolent offenses would be encouraged to participate in programs designed for rehabilitation and finish out their terms at halfway houses or with monitoring at home.

These are responsible proposals that deserve bipartisan consideration by Congress. But the problem of overincarceration will not be solved by curbing federal drug sentences. To begin with, the overwhelming majority of prisoners in the United States are incarcerated in state rather than federal prisons. And while about half of federal inmates were convicted of drug crimes, such offenses account for a much smaller proportion of state prisoners — more than half of whom, in turn, have been imprisoned for violent offenses.

For this reason, it's notable that the bill also requires the creation of a bipartisan commission to examine the criminal-justice system on the federal, state and local level. Previous proposals for such a commission have received support from both civil rights and police organizations, all of whom agree that the justice system is overdue for review. Hopefully, a National Criminal Justice Commission would be empowered to produce recommendations for reform on all levels of government beyond the simple but insufficient fix of going easier on nonviolent drug offenders.

In 2016, the Sentencing Reform and Corrections Act never received a vote due to opposition from, among others, then-Sen. Jeff Sessions. As attorney general, Mr. Sessions has since made his opposition to sentencing reform clear by advocating for prosecutors to pursue the harshest penalties possible. Even without the support of the Justice Department, Mr. Sessions's former colleagues should back this much-needed legislation.