Not long after Mayor Bill de Blasio sat beside the Rev. Al Sharpton at a July summit meeting on police reform, a political adviser gave the mayor a blunt assessment: You have a problem with the cops.

Rank-and-file officers felt disrespected by the mayor, the adviser explained, and were dismayed to see Mr. Sharpton, a longtime critic of the New York Police Department, embraced at City Hall.

But Mr. de Blasio, a Democrat, rejected the notion that officers disliked him. His message, the adviser later recalled, was clear: Everything was under control.

That confidence would last until late last month, when the murders of two officers in Brooklyn prompted the department to adopt a stance of rebellion. Uniformed officers protested against the mayor in public, and low-level arrests virtually stopped. Mr. de Blasio, a liberal who had staked his mayoralty on re-educating the police force, is struggling to secure its basic trust, the New York Times reports.

In recent days, Mr. de Blasio has sounded downcast, according to aides who have spoken with him. The manager of his mayoral campaign has returned to help. And his team is considering focus groups and a poll to refine the mayor’s message to New Yorkers who may have soured on him.