One of former President Trump's more controversial appointees, Roger Severino, is suing the Biden administration, alleging that it illegally threatened to terminate his position on a regulatory council.

"President Biden's attempt to remove me contrary to law exposes his lofty promises of healing and uniting all Americans as nothing more than cynical manipulation," Severino said in a statement provided to Fox News on Wednesday.

"Because I am not one to be bullied, not even by the President himself, I will not resign my duly commissioned post and look forward to seeing how President Biden tries to justify his vindictive actions in court."

Severino previously served as head of the Office of Civil Rights (OCR) within the Health and Human Services (HHS) Department, where he carried out much of Trump's socially conservative agenda. Before leaving office, Trump appointed Severino to the the Council of the Administrative Conference of the United States (ACUS). Severino received his commission on Jan. 16, just four days before President Biden's inauguration.

TRUMP'S CONSERVATIVE AGENDA AT HHS STALLED BY DEPARTMENT'S OFFICE OF GENERAL COUNSEL

Filed in D.C.'s district court, Wednesday's lawsuit claims that Biden cannot unilaterally end his three-year appointment to the ACUS. "The statute governing ACUS provides that Mr. Severino’s term on the Council lasts for three years, and the statute makes no provision or allowance for at- will Presidential removal," the lawsuit reads.

The suit includes the text of an email from Gautam Raghavan, the deputy director of the White House Presidential Personnel Office.

It reads: "I am writing on behalf of President Biden to request your resignation from the Administrative Conference of the United States Council by 5:00 p.m. ET tomorrow, Wednesday, February 3. If you do not resign by that time, your appointment to the Council will be terminated. Thank you for your time and service."

Severino hasn't resigned, according to the filing. He responded by telling Raghavan he was "surprised by your note. What are the grounds for the request for my resignation so early in my appointed term?"

Raghavan, President Biden, and Catherine M. Russell, who serves as director of the White House Presidential Personnel Office, were all named as defendants in the lawsuit.

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Severino claims that other ACUS members received similar emails. The White House did not immediately respond to Fox News' request for comment.

Severino's tenure at HHS included pursuit of some of Trump's more controversial policies on gender, sexuality, abortion, and religious liberty. Fox News previously reported on how former administration officials claimed that the HHS general counsel's office stalled the president's agenda at OCR.