The car was spotted near the Inauguration Day protests at Daley Plaza. View Full Caption Twiter/@madlittledebbie

CHICAGO — Chicago Police Department leaders reminded officers this weekend that they are not to participate in politics while on duty after a hat featuring President Donald Trump's "Make America Great Again" slogan was propped on the dashboard of a police vehicle assigned to monitor the Inauguration Day protests at the Daley Center.

Supervisors were instructed to call attention to rule No. 42 of the department's Rules of Conduct, which prohibits uniformed officers from "participating in any partisan political campaign or activity," Police Department spokesman Frank Giancamilli said.

A supervisor sent to investigate the SUV reported that the hat had been removed before the car was examined, Giancamilli said.

"It was addressed by supervisors to all officers that those items aren't to be displayed," Giancamilli said.

A spokesman for the police union, the Fraternal Order of Police, did not return a phone message left seeking a response.

Trump repeated the slogan at the close of his inaugural address, which he also used to cap off all of his campaign rallies. During his campaign, he sold thousands of the hats, which became ubiquitous at his events.