A veteran DC cop heralded as the first openly gay union chair and tasked with helping to protect President Trump allegedly groomed a teen boy before trying to have sex with him.

Lt. Matthew Mahl, 47, had been communicating with a detective who was posing as a teen named “Nate” while staking out a Reddit site called “r/GayYoungOldDating, NBC News reported.

Mahl — a special-operations officer whose duties included helping to provide security for Trump, Vice President JD Vance and other officials — sent sexually explicit messages and nude photos to the undercover investigator, including a “sexually suggestive” pic from the couch in his work office, authorities said.

The 23-year Metropolitan Police Department veteran even shared a picture of him in DC police uniform sitting in his cruiser, according to charging documents viewed by the outlet.

The creepy cop was unconcerned about making his identity known to the imposter teen, revealing he was a DC police lieutenant with 23 years on the job and worked in the special-events branch, according to the MPD.

He eventually gave out his phone number, which Harford County Sheriff’s Office Detective Christopher Sergent was able to link to Mahl.

The disgraced lieutenant repeatedly mentioned how “Nate” was “young and inexperienced” during their conversations and asked him if he was a sophomore in high school.

In one particularly skin-crawling exchange, Mahl wrote that he wished “it was 2/11/27 sooooo bad,” a reference to the date he believed to be the boy’s 16th birthday, at which age he’d reach the age of consent in Maryland, the documents stated.

Mahl did occasionally express fears he could “get in trouble” for carrying on with the boy and was seemingly aware of how it could impact his career.

“I have it all to lose,” he said in an apparent moment of self-reflection, according to the documents.

The conversations grew more sexually charged as the weeks progressed. Mahl allegedly sent “several explicit images of his body,” and eventually asked the detective posing as the child to send a picture of his penis and “naked self.”

The detective allegedly arranged to meet up with Mahl in Abingdon, Md., on April 14, with the undercover saying his parents would be out of town.

Mahl took the day off from work and was arrested Tuesday afternoon when he showed up at a Holford County, Md., address he was told was the boy’s house, authorities said.

An MPD rep called the allegations “extremely disturbing” and “in direct contrast to the values of … the Metropolitan Police Department,” pledging an internal investigation once the criminal investigation is complete.

Embattled DC Mayor Muriel Bowser shared her own response to the allegations Wednesday.

“We don’t have any knowledge of any accusation or any investigation involving the District, his employment or any of our children, and I don’t really know what else to say,” she said.

“We don’t want anybody like that on our force if those allegations are true.”

Ten years ago this month, Mahl was celebrated for becoming the first openly gay person to head up the DC Police Union.

He’s currently being held without bond on charges including soliciting child pornography and two counts of sexual solicitation of a minor.

His public defender did not respond to an email from The Post seeking comment.

Mahl is scheduled for a preliminary hearing May 15.