But they’ve also encountered tough headwinds. We’ve seen these new district attorneys in action, and with input from two policy groups, the Justice Collaborative and the Brennan Center for Justice, we’ve come up with a set of principles and priorities to promote a progressive model of prosecution. There are 21 principles in all that offer D. A.s a blueprint to transform both their own offices and, with a push from advocates on the outside and help from other leaders on the inside, their justice systems. Since laws and practices vary from state to state, some of our recommendations won’t suit all jurisdictions. We intend them as a starting point.

Our recommendations begin with the premise that the level of punishment in the United States is neither necessary for public safety nor a pragmatic use of resources. Prosecutors can address this first by routing some low-level offenses out of the criminal justice system at the start. For the cases that remain, they can help make incarceration the exception and diverting people from prison the rule, a principle advanced by the district attorney in Brooklyn, N.Y., Eric Gonzalez. Finally, prosecutors should recognize that lengthy mandatory sentences can be wasteful, since most people age out of the period when they’re likely to reoffend, and also don’t allow for the human capacity to change.

As prosecutors know, locking people up makes them more prone to committing offenses in the future. They can lose their earning capacity and housing, leaving them worse off, often to the point of desperation. And so the community is often better served by interventions like drug or mental-health treatment, or by restorative justice approaches, in which a person who has caused harm makes amends to the victim. In some cases, the best response is to do nothing.

Achieving results, of course, matters more than making promises. In Brooklyn last Friday, the police arrested Jazmine Headley as she sat on the floor of a food stamp application office because there were no available chairs. The officers yanked her 1-year-old son from her arms, and the D.A.’s office charged her with resisting arrest and other offenses. Although prosecutors agreed to release her without bail, Ms. Headley was held at Rikers Island on a warrant from New Jersey for credit card fraud. The arrest was captured on video and outrage ensued. On Tuesday, Mr. Gonzalez said he was “horrified by the violence” on the video, promised to investigate and moved to dismiss the charges. But this arrest shouldn’t have happened in the first place, and the response from the D.A. illustrates the back-and-forth between reformers on the outside and an elected prosecutor on the inside.

If making jail the exception in criminal cases sounds revolutionary, it shouldn’t. In many cities and counties, misdemeanors make up about 80 percent of the criminal docket. With few exceptions, locking people up for these low-level offenses, and for felonies that don’t involve serious violence or injury, is the wrong approach. The states of California, New Jersey and New York have cut the rate of incarceration by about 25 percent even as crime has fallen at a faster pace than it has nationally. In other words, locking up fewer people has correlated with making states safer, not less safe. Nationally, the population of teenagers in detention has also dropped by half alongside a major decline in the crime rate among young people. Internationally, crime is down in developed countries where incarceration always remained relatively low.