What is the main purpose of incarceration? Is it to punish people who break the law? Keep society safe? Rehabilitate offenders? Deter criminal behavior?

Congress’s latest stab at federal criminal-justice reform, officially known as the First Step Act, brings those questions to mind. Or at least it should. But proponents stress that the legislation, a version of which passed the House in May and is now under consideration in the Senate, has “bipartisan” support. That’s true, but it tells us nothing useful about the measure’s merits.

Some of what’s in the legislation would simply enforce existing rules that have long been ignored, such as housing inmates in facilities within 500 miles of their homes or families when possible. The bill would also fund new programs designed to help inmates re-enter society once their sentences are complete. None of this is controversial, and given that all but a small percentage of the incarcerated will be released at some point, focusing on re-entry programs is common sense.

What’s raised concerns among some conservatives—and what ought to give everyone pause—are measures that would reverse or soften tough-on-crime policies from the 1980s and ’90s. The Journal reported last week that the House’s version of the bill allows “some inmates to serve out the final stretch of their sentences in halfway houses or in home confinement,” while the Senate’s version gives “judges more discretion in crafting sentences in some cases and could reduce mandatory minimum sentences for some drug-related offenses.” Whoa, horsey.

There’s a reason judicial discretion was pared back in the first place. For decades, judges had been finding ways to evade public-safety measures passed by democratically elected legislators and release predatory criminals back into communities. In response, Congress and many states pushed for mandatory-minimum statutes, truth-in-sentencing laws and three-strikes provisions for the most violent offenders—all in an effort to end revolving-door justice. Making it easier for robed activists to interfere unduly with the arrest, incarceration and prosecution of lawbreakers would be a first step in the wrong direction.

President Trump said last week that he backs the legislation because it will “make our communities safer and give former inmates a second chance at life after they have served their time.” That’s possible, but the measure could just as well give former inmates a second chance at breaking the law. Most crimes are committed by people who have committed them before. A Justice Department study published in May finds “68 percent of released state prisoners were arrested within three years, 79 percent within six years and 83 percent within nine years.” Moreover, it turns out that not all “non-violent offenders” remain nonviolent. “More than three-quarters (77 percent) of released drug offenders were arrested for a non-drug crime within nine years, and more than a third (34 percent) were arrested for a violent crime,” the study concludes.

Former inmates are more likely than not to return to communities that are already overwhelmed by violent crime. How would prematurely increasing the population of ex-cons make those communities “safer”? Is the welfare of inmates more important than the welfare of the law-abiding poor who can’t afford to move out of tough neighborhoods?

Liberal proponents of criminal-justice reform play down any connection between harsher sentences and crime reduction, but there is little doubt that increasing the risk of going to prison for a crime is correlated with a reduction in the rate at which that crime is committed. Empirical research done on major crime waves that began in the 1960s and 1980s shows that would-be offenders do in fact weigh the costs and benefits of crime. Reasonable people can disagree about the size of the deterrent effect, but not whether it exists.

Those who insist that racial disparities in our criminal-justice system are driven by unfair treatment of racial minorities ignore the fact that blacks commit crimes, particularly violent crimes that draw the harshest sentences, at significantly higher rates than other groups. And they conveniently forget that back in the 1980s it was black Democratic lawmakers from urban areas wracked by the crack-cocaine epidemic—Harlem’s Charlie Rangel, Brooklyn’s Major Owens—who successfully pushed for harsher sentencing guidelines. Only in hindsight have these rules been pilloried as “racist.”

Liberals tell us they are helping the black underclass by going easier on criminals with the First Step Act, but the black poor are the primary victims of black criminals. Safe neighborhoods facilitate upward mobility. Coddling criminals—even bipartisan coddling—only facilitates more crime.