A US judge ruled on Saturday that President Donald Trump’s dismissal of the head of a federal watchdog agency was unlawful, in what is likely to become a pivotal case on the limits of presidential power before the Supreme Court.

US District Judge Amy Berman Jackson in Washington had previously allowed Hampton Dellinger, head of the Office of Special Counsel—responsible for whistleblower protections—to remain in his post while the case was under review.

In her ruling, Jackson warned that granting Trump the authority to remove Dellinger would effectively give him “a constitutional license to bully officials in the executive branch into doing his will.”

Following the decision, the Justice Department filed a notice late Saturday announcing its appeal to the US Court of Appeals for the District of Columbia.

Dellinger, who was appointed by Democratic President Joe Biden and confirmed by the Senate to a five-year term last year, welcomed the ruling. In an email to Reuters, he said he was “grateful to see the court confirm the importance and legality of the job protections Congress afforded my position.” He also vowed to continue his “efforts to protect federal employees generally, and whistleblowers in particular, from unlawful treatment.”

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Trump administration lawyers have argued that preventing Dellinger’s removal infringes on the president’s authority over officials within his administration.

However, Jackson—an Obama appointee—rejected that claim, emphasizing that the special counsel’s role is to investigate unethical or unlawful actions against federal employees and ensure whistleblowers can act without fear of retaliation.

“It would be ironic, to say the least, and inimical to the ends furthered by the statute if the special counsel himself could be chilled in his work by fear of arbitrary or partisan removal,” she wrote.

The Supreme Court, which had already delayed a ruling on the case, was urged by the Trump administration earlier in the week to intervene.

Trump has sought to limit the independence of federal agencies, including the Federal Trade Commission, the Securities and Exchange Commission, and the Federal Communications Commission. The outcome of Dellinger’s case could determine how much authority he has to do so.

Jackson stressed that her ruling was “extremely narrow” and did not weaken Trump’s executive powers. “This is the only single-headed agency left for the courts to consider, and it is unlike any of them,” she noted.

Acting Solicitor General Sarah Harris argued that Dellinger’s continued tenure was harming the Trump administration, citing his role in halting the firing of six probationary government employees the administration had sought to dismiss.

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