To end mass incarceration, however, exempting nonviolent offenses from jail time isn’t enough. People convicted of violent crimes make up more than half of the country’s state prison population. But the image of prisons overflowing with murderers and rapists is wrong. In many states, “violent felonies” include offenses like breaking into an empty house or snatching a purse or iPhone on the street. Reducing sentences for these offenses — and changing what counts as a violent felony to begin with — can lower this share of the prison population.

And that fits in with a second theme for candidates: People deserve a second chance, because many grow and change. They robbed to feed an addiction and then got sober. They assaulted someone because they were mentally ill and then got treatment and stabilized. They mature as they age beyond their teens and early 20s. That’s why it makes sense to reconsider how long a person should stay in prison after doing some time.

At last, a few prosecutors are doing this and are moving to correct the heavy sentences handed down in the past. The elected prosecutor in Seattle, Dan Satterberg, has won 21 commutations or resentencings for people serving life or decades. Three of the people released — brothers — are back in prison for new crimes, including one for a killing, but the other 18 are “taking full advantage of their freedom,” Mr. Satterberg put it.

“Our obligation to do justice continues to go backward as well as forward,” he said. “Yes, we'll have failures, but one way to manage that is more re-entry support for people.”

Parole offers another opening for second chances. In Texas, says Scott Henson, an activist who blogs at the site Grits for Breakfast, “our parole rates have gone from 15 percent to the high 30s in the last decade,” He said the increase is “having more impact than any bill we’ve passed even through the legislature.” He thinks the reason for the rise is a humdrum logistical one: The state unofficially uses parole as a way to reduce prison overcrowding.

We should also focus on redefining the terms of the public safety debate. Ending mass incarceration, and ensuring fairness throughout the criminal justice system, aren’t in tension with public safety. It is integral to it. People tend to uphold the law when they believe it’s reasonable and applied evenly. When people have that faith, they are more likely to help the police solve crimes.

With only about 60 percent of murder cases resulting in arrest or other resolutions — that percentage is even lower for other crimes — disillusionment may be our biggest safety threat. After Baltimore’s district attorney, Marilyn Mosby, announced in January that her office would stop prosecuting marijuana possession, a group of mothers whose children were killed in the city mounted a mobile billboard. “Stop Pot Arrests,” it read. “Solve Murders Instead.”