Washington (CNN) A federal judge decided Monday that President Donald Trump's former White House counsel Don McGahn must testify to the House of Representatives in its impeachment probe, marking the first major ruling about House witness testimony during the Trump administration.

"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," Judge Ketanji Brown Jackson wrote.

"Stated simply, the primary takeaway from the past 250 years of recorded American history is that Presidents are not kings," Jackson said.

The ruling is a blow to Trump and White House efforts to block parts of the impeachment inquiry. It could encourage resistant witnesses from the administration to testify and could bolster any case House Democrats make to impeach the President for obstructing its proceedings or obstructing justice.

The Justice Department has filed paperwork to appeal the judge's opinion, and McGahn's testimony may be put on hold during the appeals. The department also asked Jackson to pause her ruling pending appeal.

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