Last week, Attorney General Jeff Sessions retracted an Obama-era guidance to state courts that was meant to end debtors’ prisons, where people who are too poor to pay fines are sent. This practice is blatantly unconstitutional, and the guidance had helped jump-start reform around the country. Its withdrawal is the latest sign that the federal government is retreating from protecting civil rights for the most vulnerable among us.

The Justice Department helped shine a light on the harms of fine and fees when it investigated Ferguson, Mo., three years ago after the killing of the teenager Michael Brown by a police officer. As one of the lawyers on that case, I saw firsthand the damage that the city had wrought on its black community.

Ferguson used its criminal justice system as a for-profit enterprise, extracting millions from its poorest citizens. Internal emails revealed the head of finance directing policing strategy to maximize revenue rather than ensure public safety. Officers told us they were pressured to issue as many tickets as possible.

Even the local judge was in on it, imposing penalties of $302 for jaywalking and $531 for allowing weeds to grow in one’s yard. He issued arrest warrants for residents who fell behind on payments — including a 67-year-old woman who had been fined for a trash-removal violation — without inquiring whether they even had the ability to pay the exorbitant amounts. The arrests resulted in new charges, more fees and the suspension of driver’s licenses. These burdens fell disproportionately on African-Americans.

At the time of our investigation, over 16,000 people had outstanding arrest warrants from Ferguson, a city of 21,000. Untold numbers found themselves perpetually in debt to the city and periodically confined to its jail.

These problems were not unique to Ferguson. A Georgia woman served eight months in custody past her sentence because she couldn’t pay a $705 fine. A veteran battling homelessness in Michigan lost his job when a judge jailed him for bringing only $25 rather than the required $50 first payment to court. A judge in Alabama told people too poor to pay that they could either give blood or go to jail.

In 2015, the Justice Department convened judges, legislators, advocates and affected people to discuss this problem and devise solutions. Participants repeatedly asked the Justice Department to clarify the legal rules that govern the enforcement of financial penalties and to support widespread reform.

And so we did. Relying on Supreme Court precedent from over 30 years ago, the 2016 guidance set out basic constitutional requirements: Do not imprison a person for nonpayment without first asking whether he or she can pay. Consider alternatives like community service. Do not condition access to a court hearing on payment of all outstanding debt.

The Justice Department also provided financial resources to the field. It invested in the efforts of a national task force of judges and court administrators to develop best practices. And it created a $3 million grant program to support innovative, homegrown reforms in five states.

Along with private litigation and advocacy, these efforts have helped drive change around the country. Missouri limited the percent of city revenue that can come from fines and fees and announced court rules to guard against unlawful incarceration. California abolished fees for juveniles and stopped suspending the driver’s licenses of people with court debt. Louisiana passed a law requiring that judges consider a person’s financial circumstances before imposing fines and fees. Texas, where the court system’s administrative director said the guidance “was very helpful and very well received by the judges across the state,” issued new rules to prevent people from being jailed for their poverty. The American Bar Association endorsed the Justice Department’s guidance, and the Conference of State Court Administrators cited it in a policy paper on ending debtors’ prisons.

To justify reversing guidance that has had so much positive impact, Mr. Sessions asserts that such documents circumvent the executive branch’s rule-making process and impose novel legal obligations by fiat. Nonsense. The fines and fees guidance created no new legal rules. It discussed existing law and cited model approaches from local jurisdictions. The document also put state-level actors on notice that the department would take action to protect individual rights, whether by partnership or litigation.

Viewed in that light, the true intent of Mr. Sessions’s decision comes into focus. Sessions pulled 25 guidance documents last week. Sixteen of those involved civil rights protections — including 10 related to the Americans With Disabilities Act and one on the special harms that unlawful fine and fee practices can have for young people. Withdrawing these documents is consistent with the Trump administration’s hostility to civil rights in a host of other areas: abandoning oversight of police departments, reinterpreting anti-discrimination statutes to deny protection to L.G.B.T. individuals and switching sides in key voting rights cases.

The push to abolish debtors’ prisons will continue, as community advocates and local officials press on. It would be preferable, of course, for the federal government to fulfill its role as a leading protector of basic constitutional rights. Unfortunately, Mr. Sessions has made clear that under his leadership it will not.