Like many people in the criminal-justice system, John Chisholm, the District Attorney in Milwaukee County, has been concerned for a long time about the racial imbalance in American prisons. The issue is especially salient in Wisconsin, where African-Americans constitute only six per cent of the population but thirty-seven per cent of those in state prison. According to a study from the University of Wisconsin-Milwaukee, as of 2010 thirteen per cent of the state’s African-American men of working age were behind bars—nearly double the national average, of 6.7 per cent. The figures were especially stark for Milwaukee County, where more than half of African-American men in their thirties had served time in state prison. How, Chisholm wondered, did the work of his own office contribute to these numbers? Could a D.A. do anything to change them?

The recent spate of deaths of unarmed African-Americans at the hands of police officers has brought renewed attention to racial inequality in criminal justice, but in the U.S. legal system prosecutors may wield even more power than cops. Prosecutors decide whether to bring a case or drop charges against a defendant; charge a misdemeanor or a felony; demand a prison sentence or accept probation. Most cases are resolved through plea bargains, where prosecutors, not judges, negotiate whether and for how long a defendant goes to prison. And prosecutors make these judgments almost entirely outside public scrutiny.

Chisholm decided to let independent researchers examine how he used his prosecutorial discretion. In 2007, when he took office, the Vera Institute of Justice, a research and policy group based in New York City, had just begun studying the racial implications of the work of the Milwaukee County District Attorney’s office. Over several years, Chisholm allowed the researchers to question his staff members and look at their files. The conclusions were disturbing. According to the Vera study, prosecutors in Milwaukee declined to prosecute forty-one per cent of whites arrested for possession of drug paraphernalia, compared with twenty-seven per cent of blacks; in cases involving prostitution, black female defendants were likelier to be charged than white defendants; in cases that involved resisting or obstructing an officer, most of the defendants charged were black (seventy-seven per cent), male (seventy-nine per cent), and already in custody (eighty per cent of blacks versus sixty-six per cent of whites).

Chisholm decided that his office would undertake initiatives to try to send fewer people to prison while maintaining public safety. “For a long time, prosecutors have defined themselves through conviction rates and winning the big cases with the big sentences,” Nicholas Turner, the president of the Vera Institute, told me. “But the evidence is certainly tipping that the attainment of safety and justice requires more than just putting people in prison for a long time. Prosecutors have to redefine their proper role in a new era. Chisholm stuck his neck out there and started saying that prosecutors should also be judged by their success in reducing mass incarceration and achieving racial equality.” Chisholm’s efforts have drawn attention around the country. “John is a national leader in law enforcement, because he is genuinely interested in trying to achieve the right results, not only in individual cases but in larger policy issues as well,” Cyrus R. Vance, Jr., the Manhattan District Attorney, told me.

Chisholm reflects a growing national sentiment that the criminal-justice system has failed African-Americans. The events in Baltimore last week drew, at least in part, on a sense there that black people have paid an undue price for the crackdown on crime. Since 1980, Maryland’s prison population has tripled, to about twenty-one thousand, and, as in Wisconsin, there is a distressing racial disparity among inmates. The population of Maryland is about thirty per cent black; the prisons and local jails are more than seventy per cent black.

In 2013, former Attorney General Eric Holder announced an initiative, known as Smart on Crime, that directed federal prosecutors to take steps toward reducing the number of people sentenced to federal prisons and the lengths of the sentences. “Prison is very costly—to individuals, to the government, and to communities,” Jonathan Wroblewski, a Justice Department official who was part of the Smart on Crime team, told me. “We want to explore alternatives.” By 2014, federal prosecutors were seeking mandatory minimum sentences in only half of their drug-trafficking cases, down from two-thirds the previous year. The number of these prosecutions inched downward as well.

Last week, President Obama spoke to reporters about the criminal-justice system at the state and federal levels, saying, “If we are serious about solving this problem, then we’re going to not only have to help the police, we’re going to have to think about what can we do—the rest of us—to make sure that we’re providing early education to these kids; to make sure that we’re reforming our criminal-justice system, so it’s not just a pipeline from schools to prisons; so that we’re not rendering men in these communities unemployable because of a felony record for a nonviolent drug offense; that we’re making investments so that they can get the training they need to find jobs. That’s hard.”

The next day, Hillary Clinton gave a speech at Columbia University, in which she called for a national rethinking of the criminal-justice system, and suggested, among other things, putting body cameras on police officers. “Today, smart policing in communities that builds relationships, partnerships, and trust makes more sense than ever,” she said. “And it shouldn’t be limited just to officers on the beat. It’s an ethic that should extend throughout our criminal-justice system. To prosecutors and parole officers. To judges and lawmakers. We all share a responsibility to help re-stitch the fabric of our neighborhoods and communities.” She added, “It’s time to end the era of mass incarceration.”

Chisholm’s experiment is important on its own terms, but it is especially notable now. Scott Walker, the governor of Wisconsin and a presumptive Republican Presidential candidate, is a fierce ideological adversary. Chisholm, a Democrat, who is fifty-two, and Walker, who is forty-seven, both grew up in the state and both attended Marquette University, in Milwaukee. Both have spent virtually their entire lives working for state and local government in Wisconsin. As a state legislator, Walker pursued a traditional law-and-order agenda, and he sponsored bills that sought to increase mandatory minimum sentences for a variety of crimes. He became a statewide figure by sponsoring Wisconsin’s “Truth in Sentencing” legislation, which increased prison time and abolished parole for certain offenders. As governor, Walker has continued to oppose parole opportunities for prisoners. In 2010, the year before he took office, the state granted thirteen per cent of parole requests; in 2013, only six per cent were granted. As the Presidential campaign begins, the debate between these opposing visions of mass incarceration will play out across the nation.

Chisholm works in the Safety Building, an imposing structure built in 1930, in the days of municipal prosperity; it has terrazzo floors, and marble stalls in the bathrooms. At the time, the industrial might of Milwaukee rivalled that of Chicago, ninety miles away. But the declines of the nineteen-eighties hit Milwaukee hard, and the building looks as threadbare as the local economy. Wall clocks in the building haven’t worked since a fire in 2013. Old records are stored in the cells of an abandoned jail. (The paperwork in the case of Milwaukee’s most notorious criminal, Jeffrey Dahmer, is preserved there.)