For five years I served as a federal prosecutor, working primarily on narcotics cases and bank robberies. I loved the job and believed in the work.

However, there was an endpoint to the stories I developed and told to juries: They always concluded with the defendant going off to prison. After that slow metal gate closed behind him and I moved on to the next file, there was nothing left to tell.

Or so it seemed at the time.

In the eight years I have been teaching at the University of St. Thomas law school, I've gotten to know what happens beyond that endpoint. Part of my job has been running a clinic with my students, where we identify good cases and then petition for clemency on behalf of inmates in federal prison. Alongside my students, I have learned about all that happens after that metal gate closes, for good and bad.

More than anything, I have learned that nearly all of the people in prison come back to society at some point. And the way they re-enter society has a lot to do with their success or failure. We are all better off if they succeed.

Painstakingly, federal legislation has been crafted to move a bit in the direction of improving the chances for those leaving prison, through new programs and the creation of positive incentives. The legislation also makes minor changes to some sentencing rules, shaving off the harshest edges of the system. In a nod to its limited nature, it is titled the First Step Act.

The measure has broad support. It passed the U.S. House, has the apparent support of a majority in the Senate and a strong endorsement from President Donald Trump. In a rarity, it won praise from the editorial boards of both the New York Times and the Wall Street Journal.

So what's the problem?

Part of the holdup has been recalcitrance on the part of Senate Majority Leader Mitch McConnell to bring the bill to the floor. It appears that McConnell fears splitting his caucus of Republicans and may yield to the opposition of a small group led by Sen. Tom Cotton of Arkansas.

Cotton's opposition is rooted in the usual fearmongering that accompanies any proposal for criminal justice reform. In an Op-Ed he wrote for the National Review, Cotton criticized the First Step Act as a bill that would release violent felons into society and described the risk assessments included in the legislation as "government bureaucrats" being asked to "judge the state of a felon's soul."

Cotton's claims were so far afield that they inspired a direct response from Sen. Mike Lee, R-Utah, a staunch conservative and former federal prosecutor. Among other ripostes, Lee defended the idea of risk assessments by noting that "similar risk assessments have already been implemented in Texas and Georgia, and these states are hardly the post-apocalyptic criminal hellscapes that Cotton predicts such a system would cause."

Like Lee, I know from my experience as a prosecutor that nothing in criminal law is an exact science. Rather, criminal law works with probabilities and predictions of dangerousness, and sometimes those calculations are wrong.

I now have seen the harm that being wrong has caused in the lives of our clients at the St. Thomas clemency clinic. Many of their stories illustrate the need for the precise changes contained in the First Step Act.

Weldon Angelos sold small amounts of marijuana three times. He had a gun with him at least some of those times (though he did not brandish or use it), and that allowed prosecutors to "stack" his sentences in such a way that the judge was forced to impose a mandatory minimum sentence of 55 years on this young first offender.

Our clemency petition for Weldon was pending for four years, until he was released on an unusual motion by the prosecutor 12 years into Weldon's sentence. He's back home now, and working. I went to Utah to see him with his family, and we ate sandwiches in the backyard, sitting on folding chairs. It was powerfully mundane; freedom is like that.

The First Step Act would eliminate the ability to stack gun cases unless there is an intervening conviction, preventing outrageous sentences like Weldon's.

Ronald Blount got a life sentence for trafficking narcotics. That sounds pretty bad, but the truth is that he was a crack addict who did errands for his brother, who was selling crack. Because Ronald had two prior low-level convictions and was charged as part of a conspiracy that included all the crack his brother sold, the law required a life sentence.

President Barack Obama granted our petition for clemency and commuted Ronald's sentence. Last month, I visited with Ronald in Houston. He was taking care of his elderly mother, who was thrilled to have him home.

The law that required a life sentence for Ronald would change under the First Step Act, and would require a minimum sentence of 25 years rather than life without parole.

When sentencing and prison policy is a one-way ratchet that always moves toward longer sentences and harsher conditions, much is lost. One loss is the probability of successful re-entry. Another is the idea of human dignity. The First Step Act may be a small grace, but at least it moves in the right direction.

Mark Osler is the Robert and Marion Short professor of law at the University of St. Thomas.