Unions representing the nation’s largest police departments are pressing the U.S. Supreme Court to protect cops’ bargaining power from a possible “death spiral.”

Siding with the rest of organized labor and against the vocally cop-friendly Trump administration, nearly 20 law enforcement groups have signed “friend of the court” briefs urging the court not to outlaw the mandatory dues that pay for their bargaining and lobbying activities.

At issue is a case called Janus v. AFSCME, which has snaked its way from the lower courts in Illinois to the country's top court. Oral arguments begin on Monday, and the court is widely expected to strike down mandatory dues in a ruling by the end of June.

The case centers on Mark Janus, a state child care specialist, who objected to paying monthly dues to his union, the American Federation of State, County and Municipal Employees. “The union voice is not my voice,” Janus wrote in the Chicago Tribune. “The union's fight is not my fight. But a piece of my paycheck every week still goes to the union.”

Janus, who is represented by the National Right to Work Foundation, contends that forcing public workers to pay union dues is a violation of the First Amendment. Illinois along with New York, California and about 20 other states, requires public employees who opt out of union membership to pay the labor organization a discounted rate — called “fair share fees” or “agency fees.”

Noel J. Francisco, the Trump Administration’s solicitor general, is slated to argue in support of Janus, pointing out in its brief that “the federal government is the nation’s largest employer.”

Unions argue that these workers, nicknamed “free riders,” still benefit from the hard work of negotiating pay and benefits, and lobbying legislators for — in the case of police — public safety measures and bigger law enforcement budgets. If the “fair share” system is ruled unconstitutional, workers in union shops would be free to pay no dues at all.

The country’s largest police union, the National Fraternal Order of Police, which endorsed Trump, asserted in its brief that a ruling in Janus’ favor would lead to a “death spiral” for law enforcement.

“When individuals begin to forgo their union membership, this problem continues to compound itself,” the police union wrote. “Less members means the union must continue to make up the difference by imposing higher dues and providing reduced services...once union membership enters this tailspin, unions themselves may begin to fold.”

Similar arguments were made in a separate brief filed by the New York City Sergeants Benevolent Association, and in another filing signed by a collection of 15 police labor groups, including those representing New York detectives, officers with the Los Angeles Police Department and thousands of corrections officers across California.

“We don’t do bake sales and we don’t solicit by telephone,” said James Pasco, executive director of the Fraternal Order of Police. “We collect dues from our members.” Pasco estimated that about $11.50 of the annual dues paid by each of the FOP’s approximate 325,000 members goes to cover the union’s legal department and the national lobbying arm he oversees. An officer with “fair share” status, however, would be exempt from financing Pasco’s work thanks to a 1977 U.S. Supreme Court ruling that allows public employees to opt out of subsidizing their union’s political activities.

Despite the doomsday outcomes predicted by police union leaders in court papers, Dean Angelo, the former head of Chicago’s police union, said he is confident that officers — at least in Chicago — wouldn’t drop out of the FOP for fear of losing access to the union’s legal aid service. “It would be a very risky decision,” Angelo said. He said he could think of only one cop in Chicago who had demanded the “fair share” discount.

Some students of policing suggested that if the Janus ruling weakens police unions, that might not be altogether a bad thing. African-American policing experts said police officers of color would have a better shot at union leadership, a white male stronghold for generations. And weaker unions could mean greater accountability.

“The unions, at least in New York City, outright just protect, protect, protect the cops,” said retired NYPD commander Corey Pegues, author of a tell-all memoir, “Once a Cop." ”It’s a blanket system of covering up police officers.”