The ethics watchdog says Scavino violated the Hatch Act, which prevents executive branch officials from using their positions to influence elections.
Scavino had urged the defeat of a Michigan congressman who helped derail efforts earlier this year to repeal and replace the Obama health care law.
He tweeted April 1 from his personal account, "@justinamash is a big liability. #TrumpTrain, defeat him in primary."
The group Citizens for Responsibility and Ethics in Washington filed a complaint. Scavino was warned if he engages in such activity in the future, the office will consider it a "willful and knowing violation of the law.
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