The Phoenix city attorney sent a cease-and-desist letter to Donald Trump's campaign, insisting the Republican presidential candidate must pull a 30-second television ad because it shows him shaking hands with a group of the city's on-duty police officers.

The officers, who are featured in a clip about one second long, did not know they were being filmed and do not have permission to endorse any political candidate while on duty, Brad Holm wrote in the letter Thursday.

"In this context, the ad unmistakably and wrongfully suggests that Phoenix and the officers support or endorse Mr. Trump's campaign. That is not the case," Mr. Holm wrote, noting that the city and its police department do not endorse candidates for president or any other public office.

Trump has been endorsed by the Fraternal Order of Police, which represents some 330,000 officers nationwide, but the police in his video belong to a different union, the Phoenix Law Enforcement Association, which is part of the National Association of Police Organizations.

Neither the local nor the national union organization, which represents more than 241,000 officers, has endorsed a presidential candidate, Ken Crane, president of the Phoenix chapter, told The Arizona Republic.

City policies prohibit employees from making endorsements while on duty, wearing a uniform, or on city property, but they allow endorsements while employees are on their own time and out of uniform, the Republic reports.

While it may prove difficult to stop the Trump campaign's use of the video before Election Day, Mr. Crane said the city's letter was likely intended to discourage other campaigns, including Democratic candidate Hillary Clinton, from following Trump's lead.

“They’re trying to cover themselves, if you will,” Crane said. “Quite frankly, I think they would do the same thing if Clinton’s campaign was doing that, or at least they should."

Phoenix Mayor Greg Stanton urged the Trump campaign in a statement to comply immediately. Mr. Stanton, a Democrat who has endorsed Mrs. Clinton, has been highly critical of Trump in the past, saying his campaign's rhetoric is "in complete contrast to our values as a city."

My statement on the possible return to #Phoenix of Donald Trump & his message of division & intolerance. pic.twitter.com/UF521KQJ5V — Greg Stanton (@MayorStanton) December 10, 2015

The letter argues, furthermore, that Trump needed the city's permission before showing its police badges and insignia patches in his ad – permission the city would not give to any candidate.

"Phoenix owns the exclusive right to use these distinctive designs under federal and state law, including the U.S. Copyright Act. Phoenix strictly regulates the use of its intellectual property," Holm's letter continued. "And Phoenix does not allow any person, entity, or political campaign to appropriate or otherwise use its protected materials or replicas for any private purpose such as a campaign ad."

The Trump campaign had not responded, as of Saturday, as ABC News reported.

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The ad, which has aired in a dozen television markets, was described by The New York Times' Nick Corasaniti as Trump's "first commercial of the general election that is uplifting in tone," despite one quick clip of Clinton's "60 Minutes" interview alongside her husband in 1992.

"Until now, every ad from Mr. Trump has been negative in tone and harshly critical of Mrs. Clinton, while the candidate has claimed on the stump that his opponent was running a negative, hate-filled campaign," Mr. Corasaniti wrote. "It strikes a balance, with an energetic and uplifting feel that allows Mr. Trump to say he’s running on a positive message, while still offering his fervent supporters a negative image of Mrs. Clinton."