A police union president is blasting Mayor Bill de Blasio for his push to have New York City host the 2016 Democratic National Convention at the Barclays Center in Brooklyn.

Sgt. Ed Mullins, president of the Sergeant's Benevolent Association, called out the mayor in an open letter to the New York Post Tuesday. He said de Blasio has no business attempting to have New York City host the DNC when the NYPD feels he doesn't have its back.

Mullins says he feels the department's mission is unclear, in large part because the mayor appears to have aligned himself with protesters, more so than members of his own police force.

He said in a Skype interview that the response the NYPD has received following Eric Garner's arrest and subsequent death puts police officers in a tough situation when responding to protests, which the DNC will undoubtedly bring.

Mullins says if New York City does host the convention, officers are sworn to carry out their duties, and will protect the city as best they can. He also says it's doomed to fail, unless de Blasio reaffirms his support for the NYPD.

The mayor addressed some of the criticism at a press conference at a Brooklyn school Tuesday, and explained why the DNC would be good for the city.