National Economic Council chairman Gary Cohn, left, White House Director of Social Media, Daniel Scavino Jr., senior adviser Jared Kushner, and senior adviser Steven Miller, follow President Donald Trump to board the Marine One helicopter from the South Lawn of the White House, Wednesday, June 7, 2017, in Washington, en route to Cincinnati, Ohio. (AP Photo/Jacquelyn Martin)

National Economic Council chairman Gary Cohn, left, White House Director of Social Media, Daniel Scavino Jr., senior adviser Jared Kushner, and senior adviser Steven Miller, follow President Donald Trump to board the Marine One helicopter from the South Lawn of the White House, Wednesday, June 7, 2017, in Washington, en route to Cincinnati, Ohio. (AP Photo/Jacquelyn Martin)

WASHINGTON (AP) — White House social media director Dan Scavino violated the law when he used an official-looking Twitter account for campaign purposes, the U.S. Office of Special Counsel has concluded, issuing Scavino a letter of admonishment.

The agency also warned that if Scavino engages in prohibited political activity again, it will be considered “a willful and knowing violation of the law, which could result in further action.”

The agency concluded that Scavino, one of Trump’s most trusted aides, violated the Hatch Act, which bars most executive branch officials from using their government positions to influence elections.

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The decision came in response to a complaint from the good-government group Citizens for Responsibility and Ethics in Washington over a tweet from Scavino calling for the defeat of Rep. Justin Amash in a GOP primary.

″@realDonaldTrump is bringing auto plants & jobs back to Michigan. @justinamash is a big liability,” Scavino wrote on April 1. ”#TrumpTrain, defeat him in primary.”

Scavino’s Twitter account at the time featured photos of him standing in the Oval Office and of Trump delivering a speech behind a lectern with the presidential seal, CREW complained.

“The rules are clear that government officials aren’t allowed to use their position for campaign activity,” executive director Noah Bookbinder said in a statement. “In these overly politicized times, the separation of official and campaign business is absolutely vital in keeping partisan politics out of government business.”

Scavino is one of the president’s most loyal and longest-serving aides in the White House. He began working for Trump as a caddy at one of Trump’s golf courses, and was part of the small group of staffers who traveled with the president across the country for the entirety of the campaign.

A letter informing CREW of the decision said that Scavino was recently counseled about the Hatch Act by the Office of the White House Counsel and notes that he has not tweeted anything similar since.

Scavino did not immediately respond to requests for comment.

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