Kellyanne Conway, senior advisor to U.S. President Donald Trump, prepares for a television interview outside the White House in Washington, D.C.

The agency, which is distinct from the Justice Department special counsel's office, wrote that Conway violated the early 20th century law, known as the Hatch Act, and "ridiculed its enforcement." Conway repeatedly disparaged Democratic presidential candidates while speaking in her official capacity, the agency said.

In a letter and accompanying report sent to the president, the Office of Special Counsel wrote that Conway was a "repeat offender" whose "multiple violations of the law would almost certainly result in removal from her federal position" if she were any other federal employee.

President Donald Trump's senior counselor Kellyanne Conway should be removed from the federal government for repeatedly violating a law that bars executive branch employees from using their official position for political purposes, the government agency charged with enforcing that law said Thursday.

It is ultimately Trump's decision whether Conway keeps her job, and he is unlikely to remove her. In a statement, White House deputy press secretary Steven Groves called the Office of Special Counsel's actions "deeply flawed" and a violation of Conway's "constitutional rights to free speech and due process."

"Its decisions seem to be influenced by media pressure and liberal organizations – and perhaps OSC should be mindful of its own mandate to act in a fair, impartial, non-political manner, and not misinterpret or weaponize the Hatch Act," Groves wrote.

The White House counsel's office also took issue with the Office of Special Counsel report in a letter dated Tuesday. That letter alleged that the report was "based on numerous grave legal, factual, and procedural errors" and called the recommendation to remove Conway "as outrageous as it is unprecedented."

But some Democrats responded to the agency's report by calling on the White House to remove Conway from her job.

"Complying with the law is not optional," House Oversight Committee Chairman Elijah Cummings, D-Md., said in a statement. "Allowing Ms. Conway to continue her position of trust at the White House would demonstrate that the President is not interested in following the law—or requiring his closest aides to do so."

Cummings announced that his committee would hold a hearing with the Office of Special Counsel, to which Conway would be invited "to answer for her violations."

The agency's letter to Trump follows its March 2018 report finding that Conway violated the Hatch Act by advocating for and against candidates in the 2017 special election for Alabama's U.S. Senate seat.

The agency also cited an interview from last month in which Conway dismissed the Hatch Act.

"If you're trying to silence me through the Hatch Act, it's not going to work," Conway said, according to the Office of Special Counsel. "Let me know when the jail sentence starts," she added.

Asked for comment Thursday, Conway said, "I have no reaction. Why would I give you a reaction?"

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— CNBC's Eamon Javers contributed to this report.