Mayor Bill de Blasio’s police detail won’t be the only officers from the New York Police Department in Iowa on Tuesday.

New York’s biggest police union, the Patrolmen’s Benevolent Association, is sending about a dozen officers to protest Mr. de Blasio in Des Moines, where he is headed to burnish his image on the national stage.

“We can’t allow him to go to Iowa and try to stand on the mantle of being progressive and pro-labor when we know the truth is different,” Patrick J. Lynch, the president of the association, said in an interview. “So we’re going out there to tell our sisters and brothers: Don’t be fooled.”

Mr. Lynch said the union, which is in contract talks with City Hall, planned to have members wave signs, hand out leaflets and stand outside as the mayor delivers a speech on Tuesday night. He referred to the mayor as “the two-faced Bill de Blasio.”