A bipartisan bill to reduce sentences for nonviolent federal offenders stands atop the short list of significant legislation Congress might actually pass in this election year, but a Capitol Hill hearing Wednesday demonstrated just how tenuous even that bill’s chances are in a bitterly divided Congress.

The Senate Judiciary Committee heard testimony on a narrow but crucial issue that has emerged as the main political obstacle to a criminal justice reform bill: to what degree prosecutors must prove a defendant’s criminal intent in order to win convictions for certain federal crimes.

For most crimes, federal prosecutors must establish the defendant’s willful intent to break the law — his or her mens rea, or “guilty mind,” in its common legal formulation. But some crimes defined in federal statutes and regulations do not include willfulness as part of the elements of the offense. This means a judge or jury doesn’t necessarily need to be convinced that the accused knew what they were doing was a crime, and a coalition of mainly conservative lawmakers and activists have led an effort to write a far-ranging willfulness requirement into law.

Democratic lawmakers and liberal activists are deeply critical of this effort, calling it a quiet attempt to make it ever more difficult for the federal government to prosecute corporations and their executives for crimes against the public welfare. A top Justice Department official joined the chorus Wednesday, saying that one of the leading proposals to overhaul mens rea requirements would go far beyond corporate regulatory matters to make prosecutions of terrorists and child pornographers more difficult.

Assistant Attorney General Leslie Caldwell, chief of the Justice Department’s criminal division, said a pending mens rea proposal backed by Sen. Orrin G. Hatch (R-Utah) and House Judiciary Committee Chairman Bob Goodlatte (R-Va.) would “seriously harm our basic ability to prosecute a wide array of federal crimes” and “unleash sweeping changes across the the United States Code” that would clog courts with costly litigation and weaken the government’s ability to enforce a wide range of laws and regulations.

Caldwell recounted the 2008 terror attacks in Mumbai, India, in which four Americans were among the 164 killed. It is a federal crime to murder a U.S. national abroad, she noted, but making willful intent an element of that offense could force prosecutors to prove that terrorists knew their victims included Americans — a difficult if not impossible task, she said.

Politically, the complications are not about terrorism, but about corporate criminals. The major proponents of mens rea reform include companies, most prominently Koch Industries, and activist groups who believe that the federal government holds too much power to criminally charge companies and their executives without having to prove criminal intent.

They have the backing of such key players as Goodlatte, who declared this month that a criminal justice bill that does not broaden the need for willful intent would not pass through his committee to the House floor. But Democrats are pushing to keep the mens rea debate away from the sentencing issue, and the Justice Department’s recent pushback has raised the prospect of a presidential veto.

Caldwell, responding to a question from Sen. Richard Blumenthal (D-Conn.) on whether mens rea changes would undermine efforts to punish corporate wrongdoers, agreed that it would.

“It’s quite challenging to prove that very senior corporate executives were aware of criminal conduct that was going on within their corporations,” she said. “But … they may have consciously avoided knowing facts, and under the current law, intent can be satisfied by willful blindness.”

Hatch, the most senior Republican senator and a former Judiciary chairman, sharply rejected the notion that there should be differing standards for corporate crimes and called mens rea changes a necessary prerequisite for taking up a broader package of reforms centered on reducing sentences for nonviolent offenders.

“We must not allow the urge to do something to overwhelm the responsibility to do it well,” he said.

But several Republicans on the committee said they did not consider changes to criminal intent standards to be necessary. Two key players in criminal justice reform talks, Sen. John Cornyn (R-Texas) and Sen. Mike Lee (R-Utah), both said their supported mens rea changes but did not believe they should hold up the sentencing reform bill. And Koch Industries made clear last month after a White House summit that it would back a criminal justice bill that did not address mens rea.

Judiciary Chairman Sen. Charles E. Grassley (R-Iowa), meanwhile, argued the concerns raised by proponents of mens rea reform “just don’t hold up” and that they are “holding unrelated bills hostage.” Sen. Sheldon Whitehouse (D-R.I.) went further and suggested that the proposed changes amounted to a “Trojan horse” inserted into the criminal justice reform debate by political groups who have no broader interest in the issue.

The better approach, Whitehouse and others said, would be to consider changes to the willful intent requirements of individual law rather than sweeping changes to the criminal code.

“If we were to have the proponents of this measure actually put forth a list of the statutes they have in mind… it would not surprise me if that list showed that the vast majority of statutes are the the types of regulatory offenses where corporate defendants are most likely to be the malefactor,” Whitehouse said.

Hatch replied: “I think we ought to have in the law language that protects everybody, no matter how wealthy they may be or how poor they may be. We shouldn’t just be picking on people because we want to get a conviction.”

Senate Majority Leader Mitch McConnell (R-Ky.) would not commit Wednesday to bringing a criminal justice reform bill to the Senate floor, saying only that GOP leaders would be “working through the process of bringing everybody in the Republican Conference up to speed on this very important issue.”

Cornyn, the No. 2 Republican leader, said he hoped to strike a compromised that could satisfy Democrats who want to maintain the status quo and those like Hatch and Goodlatte who have insisted on mens rea changes.

What is becoming clear is that any process will have to move quickly to have any chance of success. With a sparse congressional schedule, presidential campaigns entering the primary season, and lawmakers starting to turn their attention to their own campaigns, leaders of both parties who have been engaged on the issue have signaled that the window for action could be closing in weeks rather than months.

“We don’t have a great deal of time to reach a compromise,” Grassley said.