The Fraternal Order of Police Lodge 5 is taking Mayor Nutter and the city's Ethics Board to court.
FOP president John McNesby said the union filed a civil lawsuit against the city last week over a decades-old rule that prohibits cops from making political donations.
The police union is the only one in the city that can't make donations to politicians or to a political-action committee.
"We're treated like second-class citizens," McNesby said. "Enough is enough."
According to the city's Home Rule Charter, cops and firefighters are specifically forbidden to contribute "whether voluntary or involuntary, for any political purpose."
Fire Fighters Union Local 22 successfully sued the city in 2003 for the right to make political contributions.
McNesby said the FOP had hoped to avoid having to sue, but months of negotiations with the city's Law and Labor departments proved fruitless.
Mark McDonald, Nutter's spokesman, said "the city will make its views on this suit known when it responds to it in pleadings before the court."