Looking to the federal government to rein in police excesses can be an exercise in managed expectations.

On Friday, Chicago agreed to revamp its Police Department after the Justice Department found routine use of excessive force, and the mayor said he would negotiate a court-enforced settlement, known as a consent decree. But that is no guarantee of results — and not just because the man most likely to be the next attorney general has said he is skeptical of such endeavors.

Attempts to force change in police departments have met with mixed success even under the Obama administration, which made police reform a signature issue. It has opened 25 investigations into law enforcement agencies over issues like excessive force, racial bias, and poor supervision, issuing reports choking with outrage.

Los Angeles, which was under a consent decree for 11 years, is regarded as one of the great success stories. “Los Angeles is a different place today because of the consent decree and the leadership of the department,’’ said Chuck Wexler, executive director of the Police Executive Research Forum, a Washington think tank. In Detroit, which emerged from a 13-year consent decree last year, officer shootings and warrantless arrests have declined significantly.

But Pittsburgh, the target of the first consent decree based on a Justice Department finding of a “pattern and practice’’ of misconduct, later backslid after leadership changes, said Samuel Walker, a criminal justice professor at the University of Nebraska at Omaha. And while Miami reduced police shootings to zero for 20 months after a 2002 federal investigation, the Justice Department in 2013 reinvestigated and found a pattern of excessive force with firearms. Miami agreed to improve supervision, training, and internal investigations without a formal decree.

The “pattern and practice’’ approach developed after the Rodney King beating in Los Angeles in 1991 forced a period of national introspection over how to curb misconduct if individual officers could not be held accountable.

Since the early attempts, Walker said, consent decrees have evolved to be more sophisticated and comprehensive. “The general pattern is that there is some backsliding on some issues,’’ he said, “but I don’t think there’s a case where a department has completely collapsed back to where it was.’’

Still, Senator Jeff Sessions, Republican of Alabama, and Donald Trump’s nominee for attorney general, called them “dangerous,’’ writing in 2008 that they “constitute an end run around the democratic process.’’ At his confirmation hearing this past week, he softened that critique, saying there were some circumstances that legitimately demanded consent decrees and that those in place would be enforced.

But, Sessions said, lawsuits could unfairly target whole police departments for the misdeeds of a few bad actors. “These lawsuits undermine the respect for police officers and create an impression that the entire department is not doing their work consistent with fidelity to law and fairness,’’ he said.

His critique did not extend to how well consent decrees work. But experts say that even systemic changes, like greater oversight of officers use force, can be slow to yield results.

“They change the ‘inputs’ through training, record keeping, community involvement and other internal reforms, but the inputs don’t necessarily translate into changes in ‘outputs’ including racial disparities, use of force, or other constitutional issues,’’ wrote Jeff Fagan, a Columbia University law professor. “The results have been quite variable.’’

Consent decrees can span years and few have successfully concluded, so it is too early to assess their effects. Several have just begun, including those in Cleveland and in Albuquerque.

Some reform advocates have expressed fears that the Trump administration will fail to investigate police departments or enforce consent decrees, robbing them of what they view as a crucial lever to compel change.

Even some police chiefs might mourn any loss of consent decrees. Baltimore’s police commissioner, Kevin Davis, has said that a consent decree would aid community relations. Chiefs may want consent decrees to insulate them from political and union opposition to change, and make it easier to demand money for reforms.