The federal prison population has more than doubled over the past 20 years, even as crime rates have plunged. Many people think that’s unjust. There is also growing agreement that it is unsafe.

Dozens of former federal prosecutors have signed on to a letter that declares: “Maintaining the status quo in federal sentencing policy threatens public safety.” It’s one of at least three such statements issued by prosecutors in recent months; federal judges and the U.S. Sentencing Commission have also called for changes.

The chief problem, as Brad Plumer has noted, is that the U.S. Bureau of Prisons is chewing up more and more of the federal law-enforcement budget, putting us on a course where punishing past crimes will be prioritized over preventing and solving future ones.

That’s true even though we're already skimping on prison spending, relative to the need. The surge of inmates has choked federal lockups to 136 percent of capacity, and that figure is on track to hit 155 percent within a decade, according to a new report from the Urban Institute. Playing sardines like this is dangerous to both inmates and guards. It also limits the rehabilitative programming that can prevent prisoners from returning to crime after their release.

To make things worse, medium- and maximum-security facilities are at 145 and 151 percent of capacity, respectively. Since they house a tougher population, these are precisely the facilities where congestion is most likely to erupt into violence.

The Senate Judiciary Committee will consider the beginnings of a fix in coming weeks when it takes up two sentencing-reform bills whose sponsors include the firebrand conservatives Rand Paul (R-Ky.) and Mike Lee (R-Utah) (in keeping with the movement’s new approach to prisons).

By allowing judges to depart from mandatory-minimum sentences under certain conditions, both bills essentially turn such sentences from requirements into recommendations. The measure Paul has championed does this for all mandatory minimums; the Lee bill simply expands a provision that already allows for judicial discretion in some drug cases. But the impact of both measures will be muted by the fact that judges rarely depart from sentencing recommendations in the first place, according to the Urban Institute.

The think tank's report estimates the impact of various reform proposals in bed-years, which represent one year in prison for one person. The researchers project that the Paul reform would save 81,000 bed-years over the course of a decade; Lee’s bill would save 53,000. That’s a lot, but not enough to put a major dent in the prison population.

This is where the Lee bill, whose other sponsors are Democrats Dick Durbin (Ill.) and Patrick Leahy (Vt.), goes further. It includes a provision to decrease certain drug-related mandatory minimum sentences by half — a change Urban calls “monumental,” with savings of 240,000 bed-years over a decade. As this chart shows, that would take a substantial bite out of overcrowding. It also translates into savings of at least $2.5 billion – a conservative estimate, because it is based on the assumption that inmates would be added at marginal cost, without the price of building new facilities to house them.

That would give almost 9,000 people a chance to be released earlier than currently scheduled, according to the U.S. Sentencing Commission. Unlike the first approach, this statutory cut in sentences does not rely on judges to be implemented.

Estimating the cumulative impact of these changes is difficult, because they have overlapping effects. For now, Urban has only plotted them out in isolation. But the bottom line is that no single option the think tank considered is likely to solve the federal crowding problem – and with it, the Justice Department’s budget problem.

“If you want to reduce the overcrowding to zero, you're going to need to do more than one of these,” said Nancy La Vigne, a co-author of the report.

What are the odds of such sweeping reform? Don't hold your breath. There is always the risk that prisons could be sucked into the bog of partisan warfare. And at a hearing last week, Judiciary Committee members Chuck Grassley (R-Iowa) and Jeff Sessions (R-Ala.) voiced skepticism about relaxing sentencing policy, pointing out that over the past two years, violent and property crime has been back on the rise.

David Dagan is a freelance journalist and a PhD student in political science at Johns Hopkins University. Follow him @DavidDagan.