Republicans in the Senate are divided over how to overhaul the nation’s criminal justice system, posing a potential setback to lawmakers who hoped to enact major bipartisan changes this year.

Sen. Mike Lee, R-Utah, remains a key player in reform efforts, actively promoting legislation that would scale back the nation’s high incarceration rates and reduce a ballooned prison population primarily through easing mandatory minimum sentencing laws.

“When we get [mandatory minimums] wrong, it affects real people,” Lee said Wednesday morning during an event hosted by The Washington Post, adding:

This robs real communities of husbands, fathers, sons, uncles, nephews, over and over and over again—communities that could benefit, families that could benefit, from their return, from their rehabilitation.

Lee, a conservative,  is working alongside two unlikely liberal allies to push the legislation to the floor: Sens. Dick Durbin, D-Ill., and Cory Booker, D-N.J.

But conservative opponents, spearheaded by Sens. Tom Cotton, R-Ark., and Jeff Sessions, R-Ala., say they fear that the changes to mandatory minimum sentencing laws would enable thousands of “violent felons” to enjoy early release from prison.

In an op-ed published Tuesday, Cotton called the bipartisan legislation “a massive social experiment in criminal leniency,” warning his colleagues on both sides of the aisle that relaxed sentencing laws could prove “devastating” to Americans.

Though only a few conservatives are publicly challenging the bill, the momentum may be enough to derail substantial change in 2016.

But Lee said he doesn’t believe that the bill is “stalled,” despite such speculation.

The Senate’s comprehensive criminal justice legislation initially was written to give judges greater discretion to reduce prison time for certain drug offenders, including some violent felons or convicts found guilty of illegally possessing a firearm.

The “Sentencing Reform and Corrections Act of 2015” sailed out of the Senate Judiciary Committee with broad support last fall. But it has yet to land on the Senate floor for debate because of divisions among Republican lawmakers.

Politico reported Monday that Republicans who helped craft the initial bill are bowing to pressure from conservative opponents, conceding to major modifications they hope ultimately will move the bill forward.

One revision would strike a proposal intended to slice mandatory prison time for “armed career criminals,” which federal law defines as those guilty of illegally carrying a firearm who also have three prior convictions for violent felonies or a record of serious drug offenses.

The bill’s authors initially reduced the mandatory minimum sentence from 15 years to 10 years. However, Politico reported that a senior GOP aide said this section has been rolled back, leaving the initial punishment intact.

Another change would slash the opportunity for inmates who had been found guilty of possessing a firearm during a violent offense or drug crime to “retroactively” qualify for a reduced prison sentence, Politico reported.

John Malcolm, director of The Heritage Foundation’s Meese Center for Legal and Judicial Studies, said the senators who opposed the initial bill were “understandably concerned” that certain provisions would reduce mandatory minimums and lead to the early release of repeat violent felons and drug offenders who had possessed firearms.

“It is one thing to reduce penalties for low-level drug dealers with minimal or no prior criminal records. It is quite another to give the same consideration to individuals who pose the greatest threat to public safety,” Malcolm told The Daily Signal.

The legislation, introduced in October by Sen. Chuck Grassley, R-Iowa, gained support from liberals such as Sen. Patrick Leahy, D-Vt., ranking member of the Judiciary Committee. The White House also endorsed it.

On Monday, The Washington Post’s “Fact Checker” column said Cotton’s repeated charge that the bill would release “thousands of violent felons” onto U.S. streets creates a “misleading impression” of a “complex” reform.

Besides Booker, Durbin,  Grassley, Leahy, and Lee, the bill’s original sponsors include John Cornyn, R-Texas; Lindsey Graham, R-S.C.; Chuck Schumer, D-N.Y.;  Tim Scott, R-S.C.; and Sheldon Whitehouse, D-R.I.

Grassley worked with Lee to ease concerns from GOP members during a briefing Tuesday evening with law enforcement officers.

Lee and Grassley argue that the legislation would enhance public safety through reduced recidivism rates while decreasing overcrowding in prisons. Lee said of critics:

When politicians argue among themselves, as we so often do about public policy questions, it can be hard to know which side is right, especially when some making arguments are not exactly wedded to the facts, and especially when some who are trying to characterize a bill have not read it.