Black pedestrians in Baltimore stopped without reasonable suspicion. Black drivers in Ferguson, Mo., searched much more frequently than whites. Cleveland residents punched and kicked by officers and subjected to stun guns, without posing any threat.

In report after report in the Obama years, Justice Department lawyers had found patterns of eye-popping rights violations and used them as leverage to force local departments to agree to major policing overhauls. But the Trump administration this week announced it was backing away from that tough-minded approach, a move that prompted fierce debate on Tuesday in cities across the country.

Many police unions welcomed the news, saying the Obama administration’s approach had impeded law enforcement and unfairly painted many good officers as wrongdoers.

The unions, a source of support for President Trump during last year’s campaign, welcomed his administration’s announcement as proof that Mr. Trump would swiftly meet his promises to restore “law and order” to the country.

But their view contrasted sharply with those of many police chiefs and politicians who have had to live under agreements struck with the Justice Department, who vowed to continue making changes to their police departments with or without the Justice Department’s imprimatur. They say that the results of consent decrees, which are backed by a court order, and so-called memorandums of agreement, which are reached out of court, have been mostly positive, if mixed, and in some cases absolutely essential.

Attorney General Jeff Sessions, who has said he doubts whether federal intervention in local departments is useful or appropriate, ordered a review of those agreements and has signaled his resistance to entering into new ones. In a March 31 memo, he assigned the task to the new deputy attorney general and associate attorney general, who are awaiting confirmation.

It is not yet clear whether Mr. Sessions will seek early termination of agreements now in effect in places like Cleveland; New Orleans; Ferguson, Mo.; Albuquerque, and Maricopa County, Ariz. The agreements, which usually expire after five years, mandate changes in training and policies like supervision, reporting contacts with the public, conducting searches, using force, de-escalating conflict, dealing with the mentally ill, and race relations.

“What Attorney General Sessions did, I think that’s just President Trump following through on his promise to help law enforcement,” said Stephen Loomis, president of the Cleveland Police Patrolmen’s Association. “And law-abiding citizens are going to benefit from it.”

Police unions, individual officers and conservatives often interpret criticism of police practices — whether from Black Lives Matter protesters, Justice Department lawyers or former President Obama — as hostility to the police. They argue that the kinds of changes demanded by the department actually hinder policing by hurting morale and making officers less aggressive, a claim that many criminologists dispute.

During the campaign, Mr. Trump picked up that theme and repeatedly referred to a major surge in violent crime — though, in fact, crime in recent years has been at some of the lowest levels ever recorded. He was endorsed by a number of law enforcement organizations, including the national Fraternal Order of Police.

In Chicago, where violence has soared in recent years, the city has announced a set of policy changes to address both crime and tense relations between the police and minority communities. In January, the Justice Department released a scathing report on excessive force and racial discrimination, and under Mr. Obama, a consent decree likely would have followed. On Tuesday, Ian Prior, a Justice Department spokesman, said he could not address what the department might do next in Chicago.

Dean Angelo Sr., president of the Fraternal Order of Police chapter that represents Chicago officers, said their problem was not police conduct, but that “they have been demonized by political leaders and others for three years straight.” Meeting last week with Mr. Sessions, “I said, ‘Our girls and guys are worried about losing their jobs for doing their job,’ ” Mr. Angelo said. “And he goes, ‘That’s not a good thing.’”

Shaun Willoughby, president of the Albuquerque Police Officers Association, said the reforms the city has made have improved police training, but he said the consent decree creates far too much paperwork for officers, limiting their policing time. And he said its restrictions on the use of force are too vague, “which leads to officers being uncertain — they’re hesitant.”

But Richard J. Berry, Albuquerque’s mayor, said that while he had some reservations —like the cost of paying court-appointed monitors — he saw the consent decree as largely positive. It may have hurt officers’ morale at first, he said, but “time and experience with it is making it less of an issue.”

The dispute illustrates the often stark divides between rank-and-file officers and the police chiefs and mayors who oversee them. For reform-minded bosses, a harsh Justice Department report and several years of court supervision can provide the political cover and authority they need to shake things up.

For community activists who complain of police abuses, and who suspect that mayors and chiefs are not sincere about reforms, federal intervention offers a way to hold local agencies to their promises.

Baltimore, subject of another blistering Justice Department assessment, reached a consent decree agreement with the Obama Justice Department, but it has not yet been finalized by a federal judge, and Mr. Sessions asked the court to hold off for 90 days.

The city’s police commissioner, Kevin Davis, pushed back hard on Tuesday, calling the requested delay “a punch in the gut to the community — certainly to me.” He suggested that Mr. Sessions may be out of step not only with people in Baltimore, but with law enforcement leaders across America.

Lt. Gene Ryan, president of the Fraternal Order of Police in Baltimore, said he recognized the value of the agreement — which, among other things, would require the city to spend more on needed equipment like laptop computers in police cruisers — but agreed with the attorney general that consent decrees are overused.

Officials in cities that are subject to Justice Department agreements, or were headed in that direction under Mr. Obama, said they would continue with reforms, no matter what Mr. Sessions does.

“The center of gravity on police reform just shifted from Washington to cities around the country,” said Chuck Wexler, the executive director of the Police Executive Research Forum, which advises departments. “But use-of-force and policing reform is not going to go away. Cities like Baltimore and Chicago still have local constituencies that are going to expect change.”

Still, some critics of policing are less hopeful, arguing that federal oversight is crucial to hold local leaders to their promises.

In Ferguson, more than two and a half years after the police shooting of a black teenager, Michael Brown, sparked weeks of unrest, “Life does not feel different for people,” said Emily Davis, a community organizer. Ferguson has been under a consent decree since last year, she said, but “They’re not treating people better.”

Ronal W. Serpas recalled that when he was the second-ranking police officer in New Orleans, the Justice Department investigated his department and identified some problems. City leaders made some changes and promised others, and no consent degree was imposed. But Mr. Serpas left the city for nine years, and when he returned in 2010 as police superintendent, he said, he found an agency that had moved sharply backward.

“While it’s fair to say that the federal government is not in the business of managing local law enforcement, they certainly are in the business of enforcing the Constitution,” he added. “And if not them, who?”