The reasoning behind the guidance centers on the fact that Mr. Trump is already running for re-election in 2020. It contends that arguments about his policies or impeachment prospects are effectively statements in support or opposition to his campaign.

“We understand that the ‘resistance’ and ‘#resist’ originally gained prominence shortly after President Trump’s election in 2016 and generally related to efforts to oppose administration policies,” the guidance said. “However, ‘resistance,’ ‘#resist’ and similar terms have become inextricably linked with the electoral success (or failure) of the president.”

And while impeachment is primarily about removing a president from office, the agency said that because a removed president would also apparently become disqualified from holding federal office in the future, supporting or opposing Mr. Trump’s impeachment amounts to taking a stand on his potential re-election.