If Congress follows through on its plan to take up criminal-justice reform next year, legislators and advocates could wrestle once again with an obscure proposal that would reshape federal criminal laws—that is, if it doesn’t imperil the reform effort first.

A bill drafted by a group of Senate Republicans earlier this year would tweak the mens rea requirement in federal statutes, adding a default rule for juries to find criminal intent for federal offenses that don’t explicitly have an intent standard. (Mens rea is a legal term derived from the phrase “guilty mind” in Latin.) If enacted, federal prosecutors would need to prove a defendant’s state of mind to obtain a conviction for a host of existing crimes. Conservatives and criminal-defense organizations argue the measure is a necessary part of the congressional effort to reform sentencing and incarceration.

But some Senate Democrats fear the measure is far too sweeping and could be a back-door attack on federal health and environmental regulations that police corporate behavior. Rhode Island Senator Sheldon Whitehouse, a member of the Judiciary Committee, told me earlier this week that he wouldn’t support a sentencing-reform bill if it included the change to mens rea. “It would turn me into a warrior against it,” he emphasized. Chuck Schumer, the Democratic leader in the Senate, would also oppose such a bill, a spokesman confirmed.

Other Senate Democrats criticized a similar measure that passed the House during the last criminal-justice-reform push, which centered on a sentencing-reform bill. In January 2016, Illinois Senator Dick Durbin, a longtime supporter of reform, said that version of the mens rea proposal “should be called the White Collar Criminal Immunity Act.” (Like Whitehouse, Durbin serves on the Judiciary Committee, which would need to sign off on any mens rea- or sentencing-reform bills.) Massachusetts Senator Elizabeth Warren said in a speech the following month that the House proposal would “make it much harder for the government to prosecute hundreds of corporate crimes—everything from wire fraud to mislabeling prescription drugs.” Negotiations over criminal-justice reform ultimately collapsed that summer as the presidential election entered its final stretch.

But with the campaign over, stakeholders in both parties want to give it another go. A bipartisan group of senators reintroduced a sentencing-reform bill earlier this month that would reduce some drug-related penalties while increasing punishments for certain violent crimes. Groups as disparate as the ACLU, the Heritage Foundation, the NAACP, and the Koch family’s foundations have teamed up in recent years to build a grand coalition for reform. But this unusual display of bipartisanship still hasn’t bridged every ideological gulf, as the brewing battle over mens rea exemplifies.

Whitehouse suggested to me that Koch-affiliated organizations may be interested in mens rea reform—and even criminal-justice reform as a whole—because of Koch Industries’ past run-ins with environmental regulators. Charles and David Koch are influential donors for Republican candidates and help fund a wide array of conservative and libertarian nonprofit groups.

“It’s an open question to what extent the Koch brothers and their operatives, in participating in the sentencing-reform conversations, had in mind all along that at a critical juncture they would try to jam their mens rea proposal into the mix,” he told me. “I would hope that that was not true, but I strongly suspect that it was.”

Mark Holden, Koch Industries’ general counsel and a prominent conservative advocate for reform, rejected that assertion. Whitehouse’s comments “demonstrate an ignorance about our efforts and their longevity,” he said in a statement. “Koch has been working on comprehensive criminal-justice reform—from sentencing to reentry—for well over a decade in a bipartisan manner at the local, state, and federal levels.”

Mens rea captures a simple principle: that a person’s intent when committing a crime should determine the punishment he or she faces for it. In modern practice, legislatures set intent requirements when drafting criminal statutes and juries determine whether defendants acted with the required state of mind to reach a verdict.

Intent standards can vary depending on the crime. Some offenses only require a jury to find the defendant acted negligently or recklessly, for example. Others are known as strict-liability offenses, for which juries can find defendants guilty without weighing their state of mind or intent. The best-known strict-liability crime is statutory rape, but it can also apply to less serious offenses like expired vehicle-identification stickers.

“It doesn’t matter whether you intended it to be expired or not, you’re held liable for it,” Norman Reimer, the executive director of the National Association of Criminal Defense Lawyers, told me. “And we accept that because it’s a minor transgression and doesn’t carry significant consequences. But if you start to hold people criminally liable under various statutes and subject them to significant penalties, then I think you get into fundamental due-process issues.”

For other crimes, intent can make a huge difference when weighing how a defendant is punished. Reimer used New York’s homicide laws as an example. “A murder is defined as killing somebody—you know, intentionally killing somebody, obviously without justification,” he explained. “Manslaughter is killing somebody with the intent of causing physical injury. Manslaughter in the second degree is recklessly killing somebody. And criminally negligent homicide is negligently killing somebody.” From negligent homicide through to murder, “the penalties go up dramatically from one to the next.”

Since 2013, Ohio and Michigan have both adopted mens rea-reform bills that added a default intent standard, meaning crimes would only be considered strict-liability offenses if explicitly described as such. But the issue is far more complex at the federal level. Unlike state legislatures, Congress doesn’t compile its criminal provisions into a holistic criminal code. Instead, federal criminal law is composed of a hodgepodge of accumulated statutes. As a result, there’s no definitive list of current criminal offenses under federal law; the lowest estimate is roughly 3,000 crimes, but some experts believe it could be far higher. (The U.S. Code, a distillation of these statutes, is technically not the official law of the United States.)

Mens rea reformers argue this vast statutory medley justifies a default standard instead of a piecemeal approach that specifies intent requirements for some crimes but not others. Earlier this month, Utah Senator and Judiciary Committee member Orrin Hatch reintroduced a bill to do just that. “Rampant and unfair overcriminalization in America calls for criminal-justice reform, which starts with default mens rea legislation,” the Republican said in a statement announcing the bill. “Requiring proof of criminal intent protects individuals from prison time or other criminal penalties for accidental conduct or for activities they didn’t know were wrong.”

Reforming intent standards has broad support from the conservative legal community, part of a broader shift there in favor of criminal-justice reform. “It’s not a huge barrier or hurdle to be proven, but it’s something that should be in there,” Holden told me. “We shouldn’t have people going to prison for things they wouldn’t necessarily know were illegal and had no knowledge or way to find out that they were.” The U.S. Chamber of Commerce and the Heritage Foundation backed Hatch’s bill, as did conservative senators like Mike Lee and Ted Cruz.

Support also came from criminal-defense organizations, a nontraditional ally for Republicans. Reimer, whose organization supports a default intent standard, framed the issue in moral terms. “The fundamental anchor of our criminal law, the moral anchor of our criminal law, is that people shouldn’t be punished unless they know they’re doing something wrong,” he told me. “That’s my elevator speech. And that’s the problem that we’ve been trying to address for many, many years now.”

However, some Democrats worry that by establishing a default intent standard, Congress would be making it harder for federal prosecutors to bring charges for regulatory offenses that currently lack an intent standard. “[This is] a category in which the public-health and safety concerns are so serious that you set out a criminal penalty as a boundary with the notion that corporations should stand well back from that boundary as part of protecting people from harm, whether it’s chemical emissions or benzene leaks or whatever it is,” Whitehouse explained.

Another notable skeptic is Chuck Grassley, who chairs the Judiciary Committee. During a committee hearing on mens rea in January 2016, he said he would be open to making smaller changes to intent standards. But the Iowa Republican rejected the broad proposal under consideration at the time. “Since strict-liability crimes do not set forth a state of mind, the House bill would change all of them to require that the defendant act ‘knowingly,’” Grassley said. “That would jeopardize public health and safety.” His office didn’t respond to a request for comment on the current bill.

“I completely and totally reject that,” Reimer said when asked about concerns that mens rea reform would help companies evade regulatory charges. He pointed to an NACDL survey of federal environmental laws that found that almost all of them, including the Clean Air Act and Clean Water Act, already establish some level of intent for criminal penalties. “I know the Federal Defenders now are endorsing it, and they’re certainly not doing it because it’s going to help polluters,” he added.

Both sides cited mens rea as one factor among many that doomed sentencing reform last year. “It all came up rather suddenly, with the world’s fastest hearing being scheduled in Judiciary and sudden declarations of the House that nothing would be done on sentencing reform without mens rea being thrown in,” Whitehouse said.

Holden lamented that the fight cost legislators and advocates momentum on pushing through the overall bill. “By the time things started to get popping again, we were in the presidential primary, which was one of the most unusual ones ever, and there was no room to get anything done,” he told me. Overall, he said, he’s optimistic about criminal-justice reform’s chances in 2018. “I had a lot of good discussions with the White House, senators, and representatives over the past year,” he told me. “And we’re hopeful something happens now, so time will tell.” Both bills have been referred to the Senate Judiciary Committee.

Some Democrats indicated they’d be open to a more limited approach on intent standards. The office of Vermont Senator Patrick Leahy, another Democrat on the Judiciary Committee, said any mens rea reform would need to be narrowly tailored, not comprehensive and retroactive. Whitehouse said he’d be willing to discuss a version that focused on “crimes in which an individual human defendant was the target,” but that other senators hadn’t taken him up on the offer yet.

As for how many of his colleagues would reject a bipartisan sentencing bill with mens rea reform in it, “I haven’t done a whip count on that,” Whitehouse told me. “I doubt very much that I’m alone on this subject, and I think I’d be even less alone if I had the chance to make the case to my caucus if it came down to this.”