House Republicans told the Department of Defense that they are “gravely concerned” about President Biden using Marines as “political props” during his Sept. 1 speech in Philadelphia.

In a letter to Defense Secretary Lloyd Austin, the lawmakers said it appeared that the Marines “participated in a political event” that “jeopardizes their compliance” with Pentagon regulations against participating in political activities while in uniform.

“We are gravely concerned that throughout the entirety of this partisan, politically charged, and divisive address to the nation, two United States Marines were visibly present flanking the President in the background,” the lawmakers wrote.

“The President’s use of active-duty Marines as political props undermines the apolitical nature of our servicemembers and erodes trust in our military,” they continued.

The letter was signed by House Republican Conference Chairwoman Elise Stefanik (N.Y.), House Armed Services Committee ranking member Mike Rogers (R-Ala.) and House Appropriations Subcommittee on Defense ranking member Ken Calvert (R-Calf.).

The Pentagon referred questions on the use of the Marines to the White House, but said “as with any letter the Secretary receives from lawmakers, his team will reply to the authors of the letter in due time.”

Biden’s use of the Marines during his speech sparked debate, with many who liked the president’s remarks criticizing the positioning of Marines behind him during a speech that was largely seen as political.

White House press secretary Karine Jean-Pierre later insisted that Biden had the Marines there out of respect for the military and its role in defending democracy.

“It is not abnormal. It is actually normal for presidents from either side of the aisle to give speeches in front of members of the military,” she added. “It is not an unusual sight or is not an unusual event to have happen.”

Pentagon spokesman Brig. Gen. Pat Ryder declined to comment on the use of the Marines during Biden’s speech on Tuesday, referring to comments made by Jean-Pierre last week.

In their letter, the lawmakers asked Austin to respond to questions regarding his knowledge of Biden’s use of the Marines during the speech.

They asked if anyone in the Defense Department knew of the contents of Biden’s speech before he gave it, if the agency knew that two active-duty service members would be involved and what guidance was given to the White House regarding their participation in the event, among other things.

“You’ve expressed your concern about politicization of the military and have insisted the military will not become a political instrument under your watch. Unfortunately, that is exactly what happened with the participation of the Marines at this political event,” the lawmakers wrote.