Mr McGahn, who left his post in October 2018, was called to appear before the House Judiciary Committee in May to answer questions about the president's attempts to impede the now-concluded Mueller investigation into Russian involvement in the 2016 presidential election.
On Tuesday, his attorney said in a statement that the McGahn ruling does not affect his situation, and he continues to seek a ruling "resolving the question whether he is constitutionally obliged to obey the House's demand that he testify or the President's conflicting demand that he decline to do so".
But Judge Jackson wrote that "the primary takeaway from the past 250 years of recorded American history is that presidents are not kings".
McGahn was subpoenaed in April by House Judiciary Committee Chairman Jerry Nadler, a Democrat from the state of NY, months before the start of the impeachment inquiry.
LUCAS: The Justice Department says that it plans to appeal today's ruling. And any decision there would likely be appealed as well.
In seeking to block McGahn and other senior White House officials from testifying, the administration made two remarkably sweeping claims, both of which would swell presidential authority to risky proportions.
As to Congress's authority to summon White House aides to testify, Jackson said the administration's assertion of "absolute testimonial immunity" has "no foundation in law" and "conflicts with key tenets of our constitutional order".
McGahn was one of the most important witnesses in former special counsel Robert Mueller's investigation into whether Trump obstructed justice as president, sitting for over 30 hours of interviews.
"Because we have adduced so much evidence of guilt of this President, so much evidence of serious misconduct, any privilege the President would have would be vitiated by this crime-fraud exception, "Schiff told CNN's Jake Tapper".
However, if this ruling stands, other Trump officials who have refused to testify in the House impeachment investigation - including former national security adviser John Bolton and acting White House chief of staff Mick Mulvaney - could be forced to sit down with impeachment investigators.
The White House has blocked, not successfully always, other officials using a similar argument. "Rather, in this land of liberty, it is indisputable that current and former employees of the White House work for the people of the United States".
The ruling is a blow to Trump and White House efforts to block parts of the impeachment inquiry.
The judge said she was not addressing the separate issue of whether McGahn could withhold information by citing executive privilege, which is meant to keep confidential the nature of discussions between a president and close aides. Cooper represents Kupperman and Bolton.
"Given that the House's impeachment inquiry is proceeding rapidly, the Committee has a finite window of time to effectively obtain and consider McGahn's testimony", lawyers for the committee said in a court filing last week.
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