The Marine Corps is calling on Rep. Duncan Hunter, R-Calif., to stop using its Eagle, Globe and Anchor emblem and trademarked phrase in campaign mailer, after both were featured in an ad that targeted two Muslim lawmakers and his Democratic challenger.

The military branch's Trademark Licensing Office sent Hunter a cease-and-desist letter on Tuesday saying the emblem and the phrase "No Better Friend, No Worse Enemy" were used in at least one mailer sent to his constituents. The letter said both may not be used by third parties without permission.

"The Eagle, Globe and Anchor is a trademark of the Marine Corps protected by federal law," Marine Capt. Christopher Harrison, a communication strategy officer, told Fox News in an email. "In accordance with 10 U.S. Code § 7881, the seal and emblem should not be used in conjunction with any political activities."

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The Marine Corps also said the trademarked phrase could imply its endorsement of Hunter's candidacy or his views on a particular issue.

The emblem and phrase recently appeared on an envelope ad that featured photos of Democratic Reps. Rashida Tlaib of Michigan and Ilhan Omar of Minnesota, the first Muslim women to serve in Congress, and Ammar Campa Najjar, who is vying to unseat the San Diego congressman for a second time.

“These three radical Democrats want you to forget their anti-Semitism or terrorist ties," a message on the mailer states. [Israel was misspelled.]

Many have called the ad Islamophobic, a charge Hunter denies, KNSD-TV reported. Omar has been the target of xenophobia in recent months for her criticism of Israel and other comments.

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Hunter spokesman Michael Harrison said the congressman is complying with the Marine Corps' request but was disappointed that "he is now being told that he cannot use this motto or image that thousands of Marines like Congressman Hunter, who went to war under this banner, have used for tattoos, coins, T-shirts, hats, books, posters and multiple other items of personal sentiment."

"It is as much a part of them as it is the Marine Corps," he continued.

Hunter, a Marine veteran who served two tours in Iraq, admitted in May to taking a photo with a dead enemy combatant.

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He is facing more urgent political battles. He faces multiple corruption charges related to the alleged misspending of $250,000 in campaign funds on personal expenses, including extramarital affairs.

His trial is expected to begin later this year. His wife, Margaret Hunter, pleaded guilty to a corruption charge in June and is cooperating with federal prosecutors.