Maurice Walker’s case is far from exceptional.

Arrested on 3 September 2015 for public intoxication, Walker, a 54-year-old black man with a serious mental disorder that leaves him unable to work, was faced with two options: pay a $160 cash bond and leave jail that day, or remain in jail over a holiday weekend. Walker told officials that he couldn’t afford the standard bond required by the city for the misdemeanor and wound up spending six days in jail, only being allowed out of his cell for an hour a day. Walker said he was also denied daily medication he took for his disorder.

“It’s definitely not unique,” said Alec Karakatsanis, one of the lawyers representing Walker in a federal suit. “What makes this case so important is that the legal issues that it raises are representative of ones that are ubiquitous in the American criminal legal system.”

Of about 11 million jail admissions every year, the vast majority of detainees are held because they cannot afford their bail.

But a few things have happened since Walker’s lawsuit was filed that have made Walker’s case potentially extraordinary, and leave it poised to represent a watershed moment in the fight against mass incarceration.

Walker sued the city of Calhoun, Georgia, where he was detained, in a class action suit, alleging that the city routinely “jails the poor because they cannot pay a small amount of money”.

Unlike in other similar lawsuits filed by Karakatsanis’s organization, Equal Justice Under the Law, the city of Calhoun didn’t settle and agree to reforms.

Walker won his case. Then the city appealed the decision, elevating it to a federal appeals court and setting up a potentially precedent-setting legal battle on the question of when and how cities may continue to jail people for being poor.

“When a case gets up to a federal appellate court it makes it much more important,” Karakatsanis said.

And then, the US justice department intervened to say it was on Walker’s side.

“Incarcerating individuals solely because of their inability to pay a fine or fee ... effectively denies equal protection to one class of people within the criminal justice system while also offending due process,” the DoJ said in a a filing to the court.

Karkatsanis said Obama’s DoJ had offered similar support in other cases, but that the significance of it coming in a federal appeals quote shouldn’t be underestimated. “It’s a real and significant announcement of the government’s position that has to go through the highest levels,” Karkatsanis said. Principal deputy assistant attorney general Vanita Gupta, one of the highest ranking members of DoJ staff, is one of the names undersigned on the brief.

The filing by the DoJ makes good on a warning letter sent earlier this year to cities across the country advising courts that they cannot jail poor people “solely because they cannot afford to pay for their release”.

In the federal system, cash bail was largely eliminated decades ago when then attorney general Robert Kennedy said pretrial detention is unfairly “directly influenced by how wealthy [a defendant] is”. The move has been supported by several subsequent supreme court rulings which have all essentially come to the same conclusion that the court did in Walker’s case:

“Any bail or bond scheme that mandates payment of pre-fixed amounts for different offenses to obtain pretrial release, without any consideration of indigence or other factors, violates the Equal Protection Clause” of the 14th amendment, the court found.

In 1964, Kennedy presented the example of a New York man, Daniel Walker, who was “arrested on suspicion of robbery and spent 55 days in jail for want of bail. Meanwhile, he lost his job, his car was repossessed, his credit destroyed, and his wife had to move in with her parents. Later, he was found to be the victim of mistaken identity and released.”

But more than a half century after Kennedy’s law went into place, jurisdictions in all 50 states routinely engage in practices that would violate the act, which only applies to suspects in the federal court system and in Washington DC. This means that today, about two-thirds of the roughly 750,000 people in jail at any one time are unconvicted, and the majority are being held for nonviolent misdemeanors.

The amicus filing by the DoJ is in line with what has been a priority of the department both under attorney general Eric Holder and now Loretta Lynch. In 2011, Holder assembled the first national symposium on pretrial justice since Robert Kennedy had done the same in 1964. The DoJ has since found various ways to try to encourage local jurisdictions to reform cash bail for poor suspects, and those efforts have been supplemented by private initiatives to urge the same.

Cherise Burdeen, who heads the Pretrial Justice Institute, said that bail reform is one of the most critical battlefronts for activists trying to stem the tide of mass incarceration. “More people will sleep in jail this year for failure to pay a fine then are in any jail or prison on a conviction,” Burdeen said.

Burdeen said that beyond being unconstitutional, cash bail simply isn’t effective, noting that the only legally acceptable purpose for the scheme is to ensure that a defendant appears for a court date, and studies show bail paid does not improve trial attendance.

A recent paper from the University of Pennsylvania also found that cash bail may actually generate more crime, as the destabilizing effects that incarceration has on the lives of poor people, for example causing them to miss work or loose a job, can push them into more precarious situations where crime becomes more attractive. The report concludes that pretrial detention “may ultimately serve to compromise public safety”.

As it stands right now, Burdeen said, the system is “detaining people who are too poor and don’t pose a threat while allowing this little trap door under the jail for people with money”. Roughly half of suspects of violent crimes are able to get out on bail nationwide, while hundreds of thousands of people facing trial for minor crimes remain locked up.

But Burdeen said the DoJ’s brief, and the general climate around criminal justice nationwide is cause for hope. “There has never been a better time for this case to be happening. It is part of a tapestry of momentum in this county on criminal justice reform, and there’s a lot of reasons for optimism.”