The federal government has issued a new guidance for the political activity of federal government workers, warning that weighing in on impeachment or talking about "the Resistance" may constitute prohibited activity, in what some ethics advocates say could be an opening to limit dissent.

The Office of Special Counsel is charged with enforcing the Hatch Act, which prohibits federal employees from engaging in political activity in the course of their work. The office, not to be confused with special counsel Robert Mueller III's investigation, is run by Henry Kerner, whom President Donald Trump nominated to the post.

The unsigned "Guidance Regarding Political Activity," which was issued on Tuesday, takes the form of a question and answer and seeks to clarify the types of actions and rhetoric considered political activity and therefore prohibited at work.

In a nod to the current climate, it stipulated that advocating for or against impeachment of a candidate for federal office would be considered political because of its implications for future elections and that any use of terms like "resistance" and "#resist" would be construed as political activity.

But some government watchdogs said they feared the guidelines could have wide-ranging effects on the nearly three million federal government employees in the United States, as well as other state and local employees who work with federally funded programmes. The ethics nonprofit American Oversight said the guidance raised "significant concerns" in a letter it sent to the office on Thursday, urging it to withdraw the memo.

"OSC's position on impeachment advocacy or opinions goes too far," the group's executive director, Austin Evers, wrote in the letter, adding that "certainly there is a difference between advocating that an official should (or should not) be elected and advocating that an official did (or did not) commit treason or high crimes and misdemeanours under the Constitution."

In particular, Mr Evers expressed concern that the guidelines could constrain whistleblowers.

"As OSC knows well, it is critically important to ensure public employees are comfortable raising concerns about waste, fraud, or abuse in the government," he wrote. "Impeachment is primarily a remedy for severe misconduct. If public employees are aware of conduct that could be impeachable but fear civil or criminal liability under the Hatch Act for saying so, they may be reluctant to approach OSA, inspectors general, or Congress."

Nick Schwellenbach, the director of investigations at the Project on Government Oversight and an employee of the OSC from 2014 to 2017, said he felt the guidance likely crossed a legal line, saying the Hatch Act was meant to be narrowly focused on political activities around parties and candidates.

"The way OSC has traditionally balanced its enforcement of that statute with the First Amendment is [focused on] supporting a candidate or political party for election. I think once you start talking about more general political views, you're starting to infringe upon people's rights," he said. "This one, I think, goes too far for them. It runs the risk of turning the OSC into an Orwellian enforcer inside the federal workforce."

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Mr Schwellenbach said he believed the guidance could be successfully challenged in court on its constitutionality.

Mr Evers also worried that the restrictions could be used to hem in employees' abilities to weigh in on policy discussions.

"If resistance terminology can become inherently 'political activity,' so too could statements like 'Build the Wall' or 'Protect our Care,' which reflect policy positions that nonetheless are tied to the president's campaign positions," he wrote. "In addition, conflating resistance terminology with electoral advocacy opens the door to public employees being retaliated against for their policy positions or opinions ... If using resistance terminology is categorically barred under OSC's Hatch Act interpretation, it could create new, insidious tools for appointees to target public employees."

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The Office of Special Counsel did not return an immediate request for comment.