Advocates for reforming the nation's criminal justice system have gotten closer than ever to changing prison conditions and sentencing laws, but they face their most high-profile test yet this week: a U.S. Senate floor fight with opponents who claim the legislation would endanger the country.

The bruising floor debate about the future of the country's criminal justice system comes after months of backroom negotiations and compromises for reformers, and represents their last chance to get the First Step Act passed before having to start over again next year.

Supporters have been upbeat, though. They have President Donald Trump at their backs, a supportive Speaker Paul Ryan, R-Wis., and a promise from Majority Leader Mitch McConnell, R-Ky., to complete debating the bill this week.

Sen. Cory Booker, D-N.J., told reporters the measure would "bring justice back to the justice system," through changes like increases to compassionate release and reducing many mandatory minimums.

"This is really significant and it is going to have a profound effect on thousands of families who have been suffering as a result of this broken system," Booker said.

However, conservatives, led by Sen. Tom Cotton, R-Ark., have balked at the legislation and called it dangerous. Cotton says it's only a matter of time before one of the prisoners released under the legislation commits another violent crime and supporters will take the blame.

He told reporters last week he intends to make sure there will be "robust" debate in the Senate and plenty of votes on amendments that will "reduce or eliminate threats to the public's safety," even if he's one of the only ones arguing against the legislation.

"If other senators want to vote for a bill that will let sex offenders and child pornographers and wife beaters out of prison, that is their prerogative and between them and the voters in their state," Cotton said.

Supporters in Congress and Jared Kushner, Trump's son-in-law, led negotiations in the last few weeks to make a flurry of changes to the bill — rolling back proposed reforms — that peeled off some of the initial opposition from conservatives. They anticipate the changes could lead to a smooth passage before the end of the year.

The bill now has a reduced "safety valve" that would allow judges to deviate from mandatory minimum sentences, and an increased number of violent offenses are excluded from "earned time" credits that would allow inmates to be released to alternative incarceration like home confinement for a portion of their sentences.

However, Cotton and others like Sen. John Kennedy, R-La., have downplayed the changes to the bill. Cotton said the legislation still goes much farther than it has to in order to pass, and should instead be pared down to just an increase in anti-recidivism programs.

"The bill still allows thousands of serious, repeat violent offenders early release," Cotton told reporters. "That is not a bill I could support."

The bill, an acronym for the Formerly Incarcerated Re-Enter Society Transformed Safely Transitioning Every Person Act, passed the House on a 360-59 margin in May, with opposition from mostly Democrats who thought the bill does 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 added 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, 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.

Proponents feel they have compromised enough already, especially to get initial Republican skeptics like Texas Sens. Ted Cruz and John Cornyn on board. Sen Dick Durbin, D-Ill., told Law360 that he was sorry to see some of the provisions go, but the bill is ready for prime time.

"I gave up more than I wanted, but I made the changes that are necessary to make sure it passed," Durbin said.

They've even gotten a handful of national law enforcement groups, like the National Fraternal Order of Police, to come 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.

The backers have also started to sound more confident about passage in recent days. Rep. Doug Collins, R-Ga., one of the main backers of the reform effort in the U.S. House of Representatives, said the negotiators have come far enough to build support for the measure and it's not the jailbreak Cotton has claimed.

If the Senate passes the bill this week, Collins told Law360 he thinks it will easily be able to pass the House before the end of the year.

"There will always be critics in the world — if you want to do something big, there will always be critics who want to stand in the way," Collins said. "This is about people and lives changing."

Kevin Ring, the president for FAMM, has been upbeat about the latest version of the legislation even if he's not happy with all of the provisions. Ring served a 15-month sentence starting in 2014 for his role in the Jack Abramoff lobbying scandal, and has argued that the targeting of the anti-recidivism programs is entirely a political decision.

The opposition from Cotton, though it has prompted some changes in the legislation, won't be able to sink the bill, Ring said.

"It's a temper tantrum, it shouldn't have much effect," Ring said. "At some point, the Chicken Little routine is going to stop working."

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Correction: A previous version of this story incorrectly stated Kevin Ring's title.

--Editing by Katherine Rautenberg.