Philadelphia has a plan to cut its prison population by one-third, or about 2,500 inmates, over three years. It just needs some help with the cost.

So said the city in a proposal submitted Wednesday to the MacArthur Foundation, seeking $2 million in grants and pledging $2 million in new and existing city money to drive its prison number down by 34 percent.

That number currently is about 7,500 inmates, in six facilities and a handful of satellite sites.

Faced with soaring jail populations nationwide, particularly for men and women awaiting trial, the foundation issued a "Safety and Justice" challenge to municipalities around the country. In May, Philadelphia was one of 20 cities to win a $150,000 planning grant from the foundation to develop strategies. Up to 10 cities will be selected for grants to implement their plans.

Philadelphia's proposal focuses on decreased reliance on cash bail, and enhanced diversion programs and mental-health supports for defendants awaiting trial on criminal charges. Currently, 75 percent of Philadelphia inmates are awaiting trial.

The city would not provide a copy of the application, citing rules of the competition. The agencies submitting the joint application included the Philadelphia Police Department, the local courts, the Defender Association, the District Attorney's Office, and the city prison system.

"The causes of mass incarceration are numerous and complex, so the fact that all our criminal justice partners have come together behind one comprehensive plan to significantly reduce our prison population is a great accomplishment," said Mayor Kenney, who in his first week in office has stressed the need for criminal-justice reform at several public appearances.

District Attorney Seth Williams said the proposal balances fairness, safety, and needed change. He said in a news release, "We'll use this proposal to ensure that the victims of crime are made whole, defendants are treated fairly, and all of us will redouble our efforts to rebuild the public's trust in our justice system."

The city expects to hear back from the foundation by mid-March.

jterruso@phillynews.com215-854-5506@juliaterruso