It's no surprise, but Philadelphia's mayor officially endorsed his fellow Democrat in the race for city district attorney. 

Larry Krasner will decrease the number of men and women behind bars in Philadelphia, said Mayor Jim Kenney.

He's "a determined and tenacious fighter.  He has a history of standing up for — and with —  the underdog," Kenney said Thursday.

Krasner, who said he will use science in criminal justice, has been traveling the country and speaking to district attorneys in a search for the best prosecutorial practices.

"The information that this campaign is obtaining doesn't belong to me, I am bringing it back to a movement in Philadelphia for balance and for fairness in the criminal justice system and especially the DA's office," Krasner said. "We are also bringing it back to all the residents in the city of Philadelphia regardless of how they vote or if they vote."

Democrats have a 7-1 voter registration advantage in Philadelphia, making it a very tough race for Republican nominee Beth Grossman.