The First Step Act is not the stuff of criminal justice reformers' dreams. The bill, which was designed in part by President Trump's son-in-law and adviser, Jared Kushner, is accurately named — a limited but not unimportant stride toward making our prison system more reasonable, humane, and just.

The bill's main concern is sentencing reform, giving judges greater discretion in sentencing for some future convictions. It also makes retroactive a prior sentencing reform law and slightly expands the circumstances under which inmates can, through good behavior and participation in educational programs, earn earlier transfer to pre-release custody, which can help them better reintegrate into society and avoid recidivism. If passed, First Step will only apply to the federal prison system, which means about nine in 10 of America's 2.1 million inmates won't be affected.

That incrementalist approach has earned First Step wide support, including that of President Trump, which means we currently have a narrow window of opportunity for meaningful, if admittedly limited, criminal justice reform at the federal level that may never open again for the next two to six years of the Trump presidency. The trouble now is Trump's interest appears to be waning.

This is not especially surprising. It has been months since Kim Kardashian West appeared at the White House in all her celebrity glory to beg Trump's mercy for Alice Marie Johnson, whose life sentence for a nonviolent drug offense he later commuted. Football players kneeling to protest police misconduct have largely faded from the headlines. Kushner may be doing his best to keep the old man on track here, but from Trump's reality TV perspective, the prison reform story arc is just about played out.

If there is any drama left in the plot, it is unfortunately to be found in opposition to the First Step Act from the right. The most strident — and dishonest — voice here is Sen. Tom Cotton (R-Ark.), a Trump administration ally with an authoritarian streak rivaled only by his enthusiasm for war.

Cotton's incredible diagnosis of the United States' criminal justice system is that it has an "under-incarceration problem," so naturally he opposes any measure that might let anyone — no matter how excessive their sentence, minimal their crime, or reformed their behavior — out of prison a little early. Cotton's fearmongering accounts of First Step's aims were so misleading that his fellow Senate Republican, Mike Lee of Utah, declared them "100 percent fake news" in a point-by-point Twitter refutation which he expanded into a searing op-ed at the conservative National Review.

While Lee, Kushner, and their bipartisan partners in Congress struggle to push ahead with the bill, Cotton likely finds a quiet ally in Senate Majority Leader Mitch McConnell (R-Ky.), who has not endorsed First Step nor, crucially, brought it to a vote. Unless that changes soon, it isn't difficult to imagine a scenario in which First Step dies with the 115th Congress on Jan. 3 and is never revived. As Sen. Lindsey Graham (R-S.C.) noted to Politico, First Step will be "a start-over project that becomes much harder" if it is not passed before this session ends.

And come January — well, who knows? Trump is infamously changeable. Given another five weeks to think about it, he may well revert to his standard punitive rhetoric and toss this reform stuff out the window. In addition to sharing the president's enthusiasm for militarism, Cotton boasts the "central-casting looks" Trump reportedly likes in his political compatriots. There would be nothing shocking about Trump — he of the apparently permanent and utterly irrational vendetta against the Central Park Five — following Cotton's siren song back to his "tough on crime" roots.

Trump is playing against type here, backing a bill that goes against his well-established enthusiasm for punishment. We are naïve to think this will last.

That precarious position makes prompt passage of the First Step Act all the more urgent. Kushner and Vice President Mike Pence deserve credit for their work lobbying GOP senators for their support, but the primary focus for First Act supporters both in and out of Congress must be pressuring McConnell to put the bill on the schedule.

The final flickers of Trump's interest in reform must be shielded, nurtured, and used before they wink out of existence, perhaps for good.