Louisiana Republican John Kennedy just added yet another twist to Senate Republicans’ heated internal fight over criminal justice reform.

Kennedy — a longstanding opponent of sentencing changes in criminal justice legislation — said he intends to block a vote on the bill this Thursday, because he’d like some more time to review it, according to BuzzFeed’s Paul McLeod.

Kennedy’s move marks the latest development in a contentious back-and-forth that has roiled Senate Republicans — who are deeply divided on the matter, despite President Donald Trump’s endorsement of the legislation.

On the one hand, there’s Sens. Chuck Grassley, Lindsey Graham, Tim Scott and Rand Paul, who are among a vocal GOP contingent pressing the Senate to support the First Step Act, a bipartisan criminal justice reform bill aimed at easing prison sentences for those incarcerated in the federal system. On the other, there’s a group of Senate Republicans, including most prominently Sen. Tom Cotton, who have vowed to oppose the legislation and argue that it could give violent criminals a pass.

The result is a split among Republicans during a time when, increasingly, everything is partisan: There is a small but dedicated group of Republicans for whom criminal justice reform feels personal. There is another subset of Republicans who worry it makes the party of “law and order” seem weak on crime. And there’s a president who, as a criminal probe into his campaign’s activities during the election circles closer, no one is quite sure where he stands despite his official support.

Because of these intra-party dynamics, Senate Majority Leader Mitch McConnell had been reluctant to bring the bill to the floor, though he announced that he would do so earlier this week.

As Vox’s German Lopez has reported, the bill itself contains a relatively mild set of reforms that would only apply to a small fraction of the broader prison population, but it would still mark one of the most significant criminal justice reform efforts that Congress has passed in years. The legislation includes key provisions aimed at cutting down recidivism and reducing mandatory-minimum sentences.

So far, it’s not clear which force in the Republican Party will win.

Jared Kushner has pushed criminal justice reform as a key legislative goal

His criminal justice support might run counter to Trump’s previous “tough on crime” campaign positioning, but the key to understanding the White House’s push for criminal justice reform begins with president’s son-in-law Jared Kushner, who has been put in charge of a lot of policies. And it’s important to remember that Kushner’s father Charles was sentenced to two years in prison for illegal campaign contributions and was released from prison in 2006.

In November, Trump gave Kushner a lot of credit for his work on the measure, calling him out during an event focusing on the issue. “He worked very hard. He feels very deeply about it,” Trump said. And earlier this week, Kushner made a rare media appearance on Fox News, in order to promote the bill.

In recent months, Kushner has vigorously pushed the case for these reforms and the fiscal benefits they offer. (An Axios report also indicated that an endorsement from the Fraternal Order of Police helped assuage some of Trump’s private concerns about how the bill would play with law enforcement interests.)

The idea of criminal justice reform has gained traction particularly among donors on the right and liberals, who have been undergoing their own evolving position on sentencing, leading the two to find some common ground over concerns that America’s exploding prison population is expensive and unjust.

Despite this, however, the bipartisan bill has rested mainly on the legislative backburner. Now, as Republicans bid farewell to their control on the house, the lame duck session appears to be one of the last opportunities to potentially get something done before Congressional dynamics shift again.

Since he’s thrown his backing behind the measure, Trump has lightly pushed McConnell for a vote before the current term ends — and even indicated that he’s open to its attachment on a final spending package that Congress needs to pass before the end of the year.

Hopefully Mitch McConnell will ask for a VOTE on Criminal Justice Reform. It is extremely popular and has strong bipartisan support. It will also help a lot of people, save taxpayer dollars, and keep our communities safe. Go for it Mitch! — Donald J. Trump (@realDonaldTrump) December 7, 2018

While Trump and others have pressed for a Senate vote on the First Step Act, a cadre of Senate Republicans, including Cotton, have bristled at the legislation and worried that it could offer early releases to those who’ve committed more violent crimes. Indeed, a talking point Cotton has pushed is that there is a “loophole” in the law that would reduce sentences for felons, even if they have longer criminal histories. (Advocates for the bill say that those who participate in rehabilitation programs need to be deemed “low-risk” before they are even allowed to qualify for “pre-release custody,” which includes programs outside of prisons that are still under close government watch.)

In the past, Senate Republicans have been concerned about the impact that backing criminal justice reform could have on their image promoting “law and order,” even as its supporters highlight potential cost savings for the government and the need for more fairer sentencing standards.

Democrats, meanwhile, appear on track to broadly support the bill. While some, including Sens. Dick Durbin and Elizabeth Warren, were initially worried the legislation wouldn’t be strong enough in its reforms — many have since relented following changes to an earlier House version that now goes further to address sentencing measures.

That means the fate of the bill comes down to what appears to be a handful of dissenting Republicans. And as of yet, it doesn’t look like they’re budging.

The Republican divide on criminal justice reform has existed for a long time

The Republican opposition toward the bill is emblematic of how a sizable wing of the party has felt about criminal justice reform for a long time: They’re not particularly interested in it and think it has the potential to make crime worse.

Cotton, one of the most vocal opponents of the bill, has argued that the legislation could lead to spikes in crime, and claimed that it would enable violent felons to cut corners on their sentences. Advocates for the bill say he’s actively misrepresenting what it can do — and note that those who benefit from rehabilitation programs would be limited to individuals who have not committed a certain set of serious crimes. Kennedy and Dan Sullivan are among other Republicans who’ve questioned the merits of the bill.

Some of Cotton’s concerns have also been echoed by the National Sheriff’s Association, which has previously pressed Trump to withdraw his support.

“According to reports the First Step Act will release dangerous criminals back into our communities without valid job-training,” Jonathan Thompson, executive director of the National Sheriffs’ Association, has said in a statement. “The bill fails to provide substance abuse treatment or individual and family counseling services needed throughout the process to a successful re-entry to society.”

If these counterpoints sound familiar, that’s because they’re the same ones Republican opponents have long used to push back against any changes to stringent sentencing measures, which they credit for reducing drug-related crimes. Cotton, for example, has even argued that the US has an under-incarceration problem, even though the country has the highest rate of incarceration of any in the world.

In 2016, when lawmakers also tried to wrangle a criminal justice reform bill, Republicans who opposed the legislation at the time including current Trump counsel Rudy Giuliani said that easing sentencing measures could be detrimental for law enforcement’s efficacy.

“Mandatory minimums and proactive law-enforcement measures have caused a dramatic reduction in crime over the past 25 years, an achievement we cannot afford to give back,” they wrote in a letter to Senate leaders.

Activists say Trump’s backing could be more aggressive

Trump himself has repeatedly signaled his support for the bill, but has only really dialed up his calls for Senate movement in recent days, as Congress’s lame-duck session rapidly comes to a close.

But even if it’s rising to the level of including it in his morning Twitter push, some activists are skeptical of his support for the bill — compared to the backing he’s expressed for other legislative measures. “He has said some supportive statements about this bill, but he has not been nearly as aggressive on pushing the advancement of this bill as he has on the wall,” the Leadership Conference’s Vanita Gupta told USA Today.

A source told Politico that Trump was, in fact, taking a “light touch” approach to ushering the bill through Congress and indicated that the president would be open to being a bit more insistent if he thinks it’s losing steam.

Proponents of the legislation argue that this could be the last chance for Congress to get something through since the new Democratic House is likely to propose much more aggressive reforms that the Republican Senate would likely oppose.

“If it doesn’t happen this year, it’s probably never going to happen,” Sen. Graham told the Washington Examiner. “Next year you’ll have Democratic control of the House, it will probably go places I can’t go, you’ll end up doing nothing.“

As things stand, however, it’s unclear whether Trump’s lukewarm statements will be enough to overcome the deeply entrenched Republican opposition.

McConnell had been hesitant about scheduling a vote on criminal justice reform. It’s because Republican buy-in was in flux.

McConnell’s reluctance to schedule a vote was due, in large part, to the split in the party. And as Vox’s German Lopez has noted, at a time when he’s facing growing pressure on a swath of policies, he wasn’t keen to prioritize legislation that divided the caucus.

“It’s extremely divisive inside the Senate Republican Conference, in fact there are more members in my conference that are either against it or undecided than or for it,” McConnell noted during a Wall Street Journal event last Monday.

Though we don’t yet have a good guess for why McConnell, who indicated as recently as last week that he doesn’t think there’s enough time to work through legislation, has changed his tune, it may have something to do with the White House’s growing pressure to consider the bill. And we know McConnell is willing to “show them a body” — meaning that he’s open to putting legislation on the floor he knows could fail.

McConnell now says the Senate might need to stay in session during the week between Christmas and New Year’s to complete its work — perhaps hoping this curbs the appetite for the bill.

At the end of the day, there are fundamental differences between the Mike Lees and Chuck Grassleys and Lindsey Grahams who argue that mandatory-minimum drug sentences are too harsh and the rate of recidivism too costly and the Tom Cottons who caution that sentencing changes and rehabilitation updates could make the party ultimately look weak on crime.

Lee has also noted that criminal justice reform could be a win for the Trump administration, since lawmakers weren’t ultimately able to get it done under President Barack Obama.

It’s currently unclear how many Republicans will truly back the bill when the vote is taken, so much so that Majority Whip John Cornyn has been criticized for the accuracy of his whip counts, according to Politico.

Its advocates believe that roughly 30 Republican senators could be supportive, Politico’s Burgess Everett has reported, while detractors like Cotton argue that the real number is much lower.