President Donald Trump's formal backing of a bipartisan criminal justice overhaul still leaves reformers facing major challenges to get the bill passed, including a tight congressional calendar and conservatives who say the changes would endanger public safety.

Surrounded by lawmakers during a White House press event, Trump on Wednesday gave his endorsement to the FIRST STEP Act, which combines a House-passed prison reform bill and sentencing changes. The move provided the bill with a strong tailwind as its sponsors navigate an uncertain path that includes either passing both the House and the Senate in the next two months, or starting over again in the new Congress.

If it passes, the bill would cut mandatory minimum sentences, give judges more sentencing discretion, increase compassionate releases for terminally ill inmates and more. Backers like Sen. Dick Durbin, D-Ill., have said that such a bill — along with its bipartisan coalition of supporters — comes only once in a long while.

"We only receive these opportunities once in a blue moon and let's not waste it," Durbin said Thursday.

Trump on Wednesday noted the rarity of such bipartisan compromise, saying "we're all better off when former inmates can receive and reenter society as law-abiding, productive citizens," and calling for Congress to pass the bill.

However, that would mean navigating a jam-packed lame duck calendar, along with pushback from conservatives who are already rallying opposition. One of the most vocal opponents of the legislation, Sen. Tom Cotton, R-Ark., told reporters Thursday that the House-passed bill has "moved consistently to the left in the Senate, even though it didn't have to," and goes far beyond what's needed.

"At the height of a drug epidemic in our country we should not be reducing sentences for serious drug traffickers," Cotton said.

The original bill, an acronym for the Formerly Incarcerated Reenter Society Transformed Safely Transitioning Every Person Act, passed the House on a 360-59 margin in May, with opposition mostly from Democrats who thought the bill did not go far enough. The original bill focused on prison conditions, such as banning restraints on inmates during childbirth, increasing "good time" credits and mandating that prisoners be held within 500 driving miles of their families.

Since then negotiators, led by Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, have sought to add sentencing law changes to the legislation. Now with a presidential endorsement, the bill includes changes to how gun charges can "stack" on one another, reductions in mandatory minimums, increased flexibility for judges to set lower sentences, changes to how weapons enhancements for drug crimes are calculated and applies prior sentencing changes from 2010 retroactively. Only those Fair Sentencing Act provisions, which reduced the sentencing disparities between crack and cocaine, will apply to current inmates.

Grassley, Durbin and other backers have acknowledged that they have a small window to get the legislation through Congress. The legislature has plenty on its calendar, including partial government funding that runs out Dec. 7 and a farm bill. Taking the time to pass the reforms would also take away from a major priority of Majority Leader Mitch McConnell, R-Ky.: confirming more than two dozen of Trump's judicial nominees.

Grassley has a more personal appeal on that front; he's calling from some "reciprocity" from McConnell for his part in shepherding more than 80 of Trump's judges to confirmation over the last two years.

"We've made history and we've got two good people on the Supreme Court and I would like some reciprocity from the leader," Grassley said.

In the meantime, Grassley and others are looking to build support for the bill in the Senate, where McConnell has said he would bring it to a floor vote if it can garner 60 votes. It already has the backing of original sponsors of the FIRST STEP Act, and reformers who favor sentencing changes like Sen. Cory Booker, D-N.J.

A handful of national law enforcement groups, like the National Fraternal order of Police, came out in favor of the new compromise. The group, in a letter from President Chuck Canterbury, cited changes they negotiated into the bill.

"Because of our engagement, the new and revised 'First Step Act' ensures that truly dangerous offenders, like those who commit crimes while armed and those who traffic in deadly narcotics like fentanyl, are ineligible for any early release programs," Canterbury said.

However other groups like The National Sheriffs' Association, Major County Sheriffs of America and Major Cities Chiefs Association called to roll back some of the gun law changes that create "a high-risk path for dangerous criminals with gun crime histories to early release from prison," in a letter to the administration and Senate leaders Thursday.

"This amounts to a social experiment with the safety of our communities and the lives of Sheriffs, deputies and police officers in the balance," the letter said.

The threat of that uncertainty reflected some of the worries of Sen. John Cornyn, R-Tex., a backer of criminal justice reform who stated Thursday that he thought the latest compromise may have gone too far to pass.

A stronger bill would focus on tried-and-true provisions that have been adopted at the state level for prison reform, according to Cornyn.

"Some of the others are more prospective in nature — we don't know for sure," he said.

--Editing by Pamela Wilkinson.

Have a story idea for Access to Justice? Reach us at accesstojustice@law360.com.