But these fixes do not reach to the heart of the problem, which is that the vast majority of federal drug offenders serving outsize sentences are in for low-level, nonviolent crimes, and have no serious history of violence.

More than half of the current drug-offender population has no violent history at all, according to a new analysis by the Urban Institute and the Charles Colson Task Force on Federal Corrections. Less than 14 percent were sentenced for using or threatening to use violence, or directing its use. And only 14 percent were sentenced for having a high-level or leadership role in a drug operation, the study found.

Virtually all of these convictions involve drug trafficking, but that can mean anything from being a kingpin to being a drug mule driving a truck to a street-corner seller. Prosecutors often rely on the threat of mandatory-minimum sentences against low-level players to go after the leaders of a drug operation. This may sound like a useful strategy, but federal research shows that other reductions in drug sentences, like the 2010 law that scaled back punishments for crack-cocaine offenses, did not make defendants less likely to cooperate with law enforcement.

Nor does it make sense to argue that long sentences for low-level, nonviolent offenders benefit public safety, since there is evidence that sentence length has little effect on crime rates.

Making any real dent in the federal prison population will require broader reforms than those Congress is currently considering. The Urban Institute has calculated that halving the sentences being served by current and future drug inmates would reduce the federal prison population by only 18 percent over existing projections by 2023.