On his last day in office, the chief federal prosecutor in Chicago made an impassioned plea for big changes to combat the city’s soaring violence, departing from the Justice Department’s usual button-down tone to criticize the local political culture, federal budget cuts and would-be reformers.

In a five-page statement issued as he resigned on Monday, Zachary Fardon, the United States attorney for Northern Illinois, outlined a plan for taking on crime and the ills of the Chicago Police Department, including a pattern of excessive force that was documented by the Justice Department in January. Most of all, he said, the Chicago police need a major increase in resources and a court-ordered consent decree, with a monitor, to make sure change occurs.

“For decades, C.P.D. has been run on the cheap,” Mr. Fardon wrote. “Officers don’t have the training, the supervision, the equipment or the culture they need and deserve.

“If you leave correcting those deficiencies to the vagaries of city politics, then you likely lose the long-term fight.”

Chicago police officials outlined a plan for improvement on Tuesday that they pledged to follow even if the Justice Department does not pursue a consent decree. Superintendent Eddie Johnson said that the police training program would be overhauled, that supervision would be improved by having fewer officers assigned to each sergeant, and that a new use-of-force policy would soon be finalized.

“We’re not just saying we’re going to reform — we’re showing you that we’re reforming,” Mr. Johnson said. “If you go out there right now, C.P.D. is different than it was this time last year. So we don’t need a piece of paper to ensure that we do it. We’re doing it.”

Mr. Fardon was one of 46 United States attorneys whom the Trump administration told on Friday to resign immediately. The suddenness was unusual; new presidents routinely remove the prosecutors but often keep them on while searching for their replacements.

Preet Bharara, the United States attorney in Manhattan, refused to step down and was fired. His counterpart in Montana, Michael W. Cotter, resigned but called the administration’s handling of the matter “very unprofessional.” A few others called it abrupt and unnecessary.

Mr. Fardon’s parting message was different, raising no objection to his ouster. Instead, it was a call to address a surge of violence in Chicago that has defied the efforts of police officers who “in their quiet moments struggled with their own sense of frustration and despair.”

Mr. Johnson called Mr. Fardon “a great partner” to the Police Department and said he agreed with Mr. Fardon’s call for more prosecutions on federal gun laws. He also addressed Mr. Fardon’s claim that police officers had become hamstrung and too passive on patrol.

“Listen, there are things within C.P.D. that we did need to fundamentally change, and we’re changing them,” Mr. Johnson said, adding that officers were focusing on arresting gun offenders and other serious criminals.

“I think that the rank-and-file understand that we have a job to do,” he said. “You know, we swore an oath to protect the citizens of the city, and that’s what we’re going to do.”

Donovan Price, an anti-violence activist who agreed with parts of Mr. Fardon’s statement, said, “I believe he shed some tears when he wrote this.”

Consent decrees and federal oversight, favorite tools of the Obama administration and police critics, are opposed by many conservatives, including Attorney General Jeff Sessions. Mr. Fardon criticized budget cuts that have thinned the ranks of federal prosecutors. He called for a significant increase and better cooperation among federal law enforcement agencies.

But he also said that criticism of the police, and efforts to restrain them, had undermined morale and contributed to the rise in crime — a claim often made by conservatives but rejected by liberal advocates of police reform.

Mr. Fardon took particular aim at a 2015 agreement between the police and the American Civil Liberties Union of Illinois that sharply curtailed the number of people — primarily black men — the police could stop on Chicago’s streets. He wrote that the settlement had told officers, “If you go talk to those kids on the corner, you’re going to have to take 40 minutes to fill out a form.”

He said that change, along with the firing of a police superintendent and the release of a video showing an officer shooting a 17-year-old, Laquan McDonald, 16 times, had created a morale crisis. As officers made fewer stops, he said, “kids started shooting more.”

Karen Sheley, director of police practices for the A.C.L.U. of Illinois, said Mr. Fardon had overstated the burden of the 2015 agreement, adding, “Trying to blame reform for a rise in crime is a mistake.”

Many cities had increases in violent crime last year, but Chicago stood out as homicides jumped more than 50 percent, to 762, more than New York and Los Angeles combined.

Mr. Fardon wrote that crime-plagued communities needed a far bigger police presence and more programs to divert young people from gangs. And he called for an aggressive crackdown on social media, where people air grievances that escalate to gunfire.

These prescriptions require money, so their prospects are unsure, at best, in a city and state with serious financial challenges and under an administration in Washington that has promised to cut spending.

Jim Bueermann, president of the Police Foundation, a law enforcement research group, praised some of the ideas and was hesitant about others. But he said it was important that Mr. Fardon had aired them.

“There are a lot of people that work in the criminal justice system that feel deeply and personally about these issues, but the public doesn’t hear that very often,” Mr. Bueermann said. “I think more prosecutors and police chiefs should do what he did.”