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 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 announced this week that 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 been living under agreements struck with the Justice Department, who vowed to continue making changes to their police departments with or without the 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.