President Donald Trump’s support has put Congress within reach of passing the most sweeping set of changes to the federal criminal justice system since the 1990s, when fear of crime drove the enactment of draconian sentencing practices that shipped hundreds of thousands of drug offenders to prison.

This is no small feat. Reformers have been trying to get this done for years, but something always got in the way: partisan bickering, election-year politics, ambushes by opponents. Amid Washington gridlock, the First Step Act stands out.

The measure, which could go to a vote during the lame-duck session of Congress between now and January, contains several changes to the way the federal government treats drug offenders, both those who are in prison now and those who will face a judge in the future.

If it is passed, thousands of federal prisoners would have access to more help preparing for life after the end of their sentences. Thousands of well-behaved prisoners would win freedom earlier. And thousands of people who are arrested for drug crimes in the future would become eligible for exemptions from harsh mandatory minimum sentencing laws.

Still, the bill stops short of what many reformers say is needed to curb prison spending, relieve staff shortages and overcrowding, and make the justice system fairer. That is why they say the First Step Act is exactly that: a start.

It’s also worth noting that the federal prison system, with 183,000 inmates, is tiny in comparison to America’s total incarcerated population of 1.5 million. State prison systems make up the vast majority of that population, and that is where reform efforts have been spreading for years. Many states, both red and blue, have cut their prison populations while also seeing drastic reductions in crime.

“The only reason we’ve gotten to the point of these federal reforms, which we think will be transformative, is the fact that states have applied them and have data they can point to that can persuade the tough-on-crime crowd,” said Mark Holden, chairman of Freedom Partners, which makes conservative arguments for criminal justice reform. “It is hard to argue against that, and it’s all based on data.”

Here is what the First Step Act would do:

Give some prisoners a break

Nearly a decade ago, Congress addressed one of the most glaring inequities of the drug war-era sentencing laws, which punished people convicted of crack cocaine-related offenses (most often poor residents of urban areas) much more harshly than those convicted of powder cocaine-related crimes. The Fair Sentencing Act of 2010 reduced that disparity. The First Step Act would apply that correction to prisoners who were sentenced for crack offenses before the 2010 law passed. There are nearly 3,000 such prisoners, many of whom would become eligible to petition for their release immediately.

The First Step Act also changes the way the U.S. Bureau of Prisons calculates the amount of “good-time credit” for prisoners who stay out of trouble behind bars, adding seven days of credit per year. Applied retroactively, that tweak means that an estimated 3,900 prisoners would be eligible to leave prison earlier in the first year the changes take effect.

The First Step Act also gives prisoners an incentive to participate in training and educational programs by awarding them points that speed their release to halfway houses or home confinement.

Those programs would expand and improve under the bill, which allocates millions of dollars for the Bureau of Prisons to develop them.

The bill exempts several categories of prisoners from earning more credits, including convicted murderers, sex offenders, terrorists and spies, as well as immigrants who crossed into the U.S. illegally and people convicted of trafficking fentanyl.

Ease harsh sentences

In addition to helping certain people who are behind bars, the First Step Act addresses some aspects of the system’s “front end” — the laws that guide how prison sentences are determined.

One addresses “safety valves” that allow judges to diverge from mandatory minimum sentencing laws. The current system allows judges to give lower sentences to nonviolent drug offenders with little or no prior criminal history; the First Step Act would allow judges to apply those safety valves to nonviolent drug offenders with longer criminal histories. This provision would benefit about 2,100 offenders annually, according to the Congressional Budget Office.

Giving judges more discretion over sentencing decisions "is probably the most significant piece of the legislation, and the one we were most concerned about staying in the bill,” said Holly Harris, a longtime Republican strategist who leads the U.S. Justice Action Network, which recruits lawmakers on the left and right to overhaul the criminal justice system.

Another piece of the First Step Act would restrict a common practice among prosecutors to seek longer sentences by adding gun charges against a drug defendant even if the gun wasn’t used or fired. The law was meant to target repeat offenders, but it has been used to “stack” charges against first-time offenders, adding up to 25 years to their sentences. This would affect about 60 defendants a year, reducing their potential sentences by about half, the Congressional Budget Office has said.

Another 60 defendants a year would benefit from a reduction in mandatory minimum sentences for repeat serious drug offenders, including the elimination of life without parole for someone with three or more prior drug offenses.

Those three sentencing changes were retroactive in prior versions of the bill, but in the new version they will only apply going forward — a concession to bring more skeptical lawmakers on board, said Sen. Chuck Grassley, R-Iowa, one of the measure’s authors.

“It’s probably true that each of us could find something to disagree with in this bill,” Grassley said in a statement Thursday. “But an opportunity to pass legislation like this that does so much good does not come often. And when it does come, we must join together and support it.”

The First Step Act is considered a potentially landmark piece of legislation, the result of years of lobbying by advocates inside and outside of Congress. But the bill doesn’t cure all of the system’s shortcomings, they say.

Kara Gotsch, director of strategic initiatives at the Sentencing Project, a reform-advocacy organization, said she saw the First Step Act as progress, but it left “so much more do to.”

“It doesn’t end racial disparity in the system. It doesn’t dramatically reduce the prison population,” Gotsch said. “Every little bit helps, but it’s not the end of the conversation.”