A federal judge on Monday ordered former White House Counsel Don McGahn to testify before the House Judiciary Committee on a subpoena, a ruling the Trump administration said it would appeal.

Judge Ketanji Brown Jackson sided with Congress in the first lawsuit House Democrats filed to force an official to testify since President Donald Trump announced a fight-all-the-subpoenas strategy.

The committee sought to enforce its subpoena of McGahn to testify about what he told investigators in former Special Counsel Robert S. Mueller III’s probe into potential obstruction of justice by Trump.

She described the Trump administration’s argument that McGahn has “absolute immunity” from congressional subpoenas as “a fiction” that “simply does not exist.”

“That is to say, however busy or essential a presidential aide might be, and whatever their proximity to sensitive domestic and national-security projects, the President does not have the power to excuse him or her from taking an action that the law requires,” Jackson wrote.

But the judge did not settle which questions McGahn might not answer while testifying.

Jackson in her opinion distinguished that from “the very different question of whether the specific information that high-level presidential aides may be asked to provide in the context of such questioning can be withheld from the committee on the basis of a valid privilege.”

DOJ spokeswoman Kerri Kupec said Monday night that the administration will appeal.

While the focus of the impeachment inquiry has moved to Trump’s dealings with Ukraine, some House Democrats consider the McGahn case the key test of congressional subpoena power for other ongoing investigations.

House Democrats still want McGahn to play a role in the impeachment process. The Judiciary Committee anticipates holding hearings after the ongoing House Intelligence Committee hearings have ended.

The Judiciary Committee “would aim to obtain Mr. McGahn’s testimony at that time,” House General Counsel Doug Letter told Jackson in a court filing last week.

Yet that Judiciary Committee portion could come as soon as December. An appeal of Jackson’s ruling, if it escalates to the Supreme Court as anticipated, could take weeks and maybe months to resolve.

The Judiciary Committee says the Mueller report on Russian interference in the 2016 election mentions McGahn’s statements 160 times in the section that focuses on whether Trump tried to obstruct the investigation.

And they argue he can give lawmakers a firsthand account of his discussions with Trump and other White House aides about the president’s actions and their reactions to them.

McGahn’s lawyers argue he is “absolutely immune” from compelled testimony about his official duties as counsel to Trump, because confidential advice is essential to a president’s effective performance.