In June, Representatives Jim Sensenbrenner (R-WI) and Bobby Scott (D-VA) introduced the most ambitious criminal-justice reform bill Congress had seen in years. And on July 16, Speaker of the House John Boehner promised he'd give it a vote on the House floor.

The bill, called the SAFE Justice Act, bundles together a lot of relatively small reforms into a package that could reduce the federal prison population significantly. It's built on recent criminal-justice reforms taken at the state level, as dozens of states have cut prison sentences and adopted "data-driven" rehabilitation policies.

Why the SAFE Justice bill has a chance to pass the House

Over the past couple of years, momentum on criminal justice reform has been in the Senate, which has considered bills such as the Smarter Sentencing Act (to cut congressionally mandated minimum sentences) and the Corrections Act (to allow some inmates to earn time off their sentences while in prison). And the Senate is still negotiating a potential bill that would combine elements of both of these proposals. But the House has proposed its own solution in the SAFE Justice Act, which is bigger than either Senate bill — it includes variations on both of them, as well as a lot of other things — but also a little more politically cautious. And criminal justice reformers on both the left and the right are extremely excited about it.

While Senate efforts at criminal justice reform have exposed a generational split in the Republican Party, in which young reformers like Senators Mike Lee and Rand Paul face off against old-school, tough-on-crime conservatives like Senators Chuck Grassley and Jeff Sessions, the House's bill was written by one of those old-school Republicans — Rep. James Sensenbrenner of Wisconsin — as well as Rep. Bobby Scott (D-VA).

Sensenbrenner and Scott think of the SAFE Justice Act as a federal version of the criminal justice reform bills that have been taken up in state after state over the past several years, many of them under the mottos of "justice reinvestment" and "smart on crime." In their minds, they're building on what's worked in the states and are in line with reformers' emphasis on "data-driven" and "evidence-based" criminal justice policymaking.

No one knows exactly how much this will cut prison sentences — but it would be significant

The SAFE Justice Act is a collection of dozens of different reforms. Most of them aren't terribly big on their own, but many of them overlap. That makes it really hard to estimate exactly how much the federal prison population would shrink if the bill became law. But its effect would be bigger than anything that's been introduced in Congress so far.

Many of the reforms would cut sentences for drug crimes — which reflects a growing consensus that nonviolent drug offenses aren't as bad as violent crimes. Drug prisoners are about half of all federal prisoners (unlike in states, where violent crime is the biggest cause of incarceration). That means that many of the SAFE Justice Act's biggest reforms would target the largest slice of the federal population.

Of course, many more people are in state prison than in federal prison. So the SAFE Justice Act isn't enough to fix incarceration nationally. But then, no federal bill would be.

Fewer drug prisoners, and in prison for less time

Most changes to prison sentences in Congress have focused on cutting mandatory minimum sentences, which force judges to sentence people to five, 10, or 20 years for certain drug crimes. But across-the-board cuts to mandatory minimums have been met with serious resistance from old-school Republicans, including Senate Judiciary Chair Chuck Grassley (R-IA).

The House's solution, via the SAFE Justice Act, isn't to reduce the mandatory minimums themselves — but to narrow the range of people who they apply to. Instead of someone who's convicted of trafficking a certain amount of cocaine being automatically sentenced to 10 years, for example, he'd only trigger the 10-year minimum if he were also a leader or organizer of an organization of five or more people. And even then, the bill says that judges can override the mandatory minimum if the defendant doesn't have much of a criminal history, or has a serious drug problem.

The bill would also make it possible for more people to be sentenced to probation instead of getting sent to prison. It would allow drug offenders to get probation if they'd been convicted of low-level drug crimes before. It would encourage judges to give probation to first-time low-level offenders. And it would encourage districts to start up drug courts and other "problem-solving courts"; some states have found these are better ways to treat some addicts than prison is.

This gives judges a lot more authority to determine how much prison time someone actually needs to serve, given the particulars of his case — or whether he needs something else entirely.

Prisoners could get 33 percent off their prison terms for participating in programming

Current prisoners whose sentences would have been affected by the bill's front-end reforms could apply to get their sentences reduced that way. But the SAFE Justice Act would also give them another way to reduce their sentences: by getting time off for rehabilitation.

Under the bill, every federal prisoner would get an individual case plan, based on what particular prison education, work, substance abuse, or other programs are the best fit for his needs. For every month a prisoner follows the case plan, he'd get 10 days off his prison sentence — meaning a prisoner with a perfect behavior record could get his sentence reduced by a third. (Prisoners serving time for homicide, terrorism, or sex crimes aren't eligible for time off, but that's a very small slice of the federal prison population.) The logic is that prisoners who want to rehabilitate themselves, and whose good behavior shows they're succeeding, shouldn't be forced to spend extra time in prison just for prison's sake.

The bill goes even further when it comes to probation — which affects many more people than prison. For every month of perfect behavior on probation, the offender would get 30 days off the end of his sentence — essentially cutting the probation term in half. If the offender violated probation, on the other hand, there would be a set of gradually escalating punishments, instead of an automatic ticket back to prison.

It's not a policing reform bill — but it tries to direct money toward community policing

Supporters estimate that after a few years to get its programs set up, the SAFE Justice Act would start saving the government serious money on new prison spending — about $2 billion per decade.

Sensenbrenner and Scott want some of that money to be redirected into spending on local police. There's a policy reason for this: Research shows that potential criminals aren't deterred by the thought of an especially long prison sentence, but they are deterred by the knowledge that they'd definitely get apprehended. So the bill would redirect deterrence efforts from prisons to policing.

The point of supporting local police is to make it easier for them to engage in community policing — which is hard to do well when police don't have enough people, or when they're under pressure to make money by issuing as many tickets as possible. It's not a comprehensive policing reform bill; it's more a list of suggestions than anything else. That might not be a bad thing, politically: In the wake of the Black Lives Matter movement, there's deep partisan disagreement on how the federal government should treat police. But reforming prisons has remained a bipartisan issue, even as policing has gotten polarized. The SAFE Justice Act reflects that.

The question: will it become law?

In the year 2015, it is extremely hard to get any sort of bill through Congress. And Sensenbrenner, Scott, and their fellow reformers have a narrow window before the presidential campaign saps Congress of any will to act it has left. So the barriers are pretty high.

But this isn't, in itself, supposed to be a polarizing bill. The presence of Sensenbrenner and other old-school Republicans reflects that. And this is something that both houses of Congress have been debating for some time.

If Speaker Boehner keeps his promise to bring the SAFE Justice Act to the floor quickly, it might give the Senate enough time to consider the bill itself. Or perhaps the Senate's pending criminal-justice bill will go into conference with the House's version. But either way, the fact that an ambitious, bipartisan criminal-justice bill is going to get a floor vote in the House of Representatives is significant — and it's something that it's impossible to imagine happening a few years ago.