Democrats filed the lawsuit against McGahn to force him to testify in the obstruction of justice issue relating to the Russian Federation probe, but the ruling could also impact other areas such as the impeachment process, where key Trump administration officials have declined to testify under direction from the White House.
The ruling, issued by U.S. District Judge Ketanji Brown Jackson, asserted that the Trump administration can not block McGahn from obeying his House subpoena under claims of immunity.
Jackson declared in her 118-page ruling that "presidents are not kings" and that "no one is above the law".
The ruling came in a case brought by the House Judiciary Committee in August in an effort to enforce its subpoena demanding McGahn's congressional testimony about events relating former special counsel Robert Mueller's investigation after President Donald Trump directed him not to comply.
According to the Mueller report, McGahn told Mueller's team that Trump repeatedly instructed him to have the special counsel removed and then asked him to deny having been so instructed when word of the action emerged in news reports.
President Donald Trump claimed he would "love" to have Secretary of State Mike Pompeo, acting chief of staff Mick Mulvaney and other top officials testify in the Democratic impeachment inquiry.
The Trump administration is expected to appeal, which could delay McGahn's testimony to House investigators. They cited a Justice Department Office of Legal Counsel opinion that top presidential advisers are absolutely immune from testifying before Congress. "No one, not even the head of the Executive branch, is above the law".
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Mark Milley said at the time that military leaders were protecting the dog's identity. "Nobody's going to mess with Conan". It was a flawless attack and al-Baghdadi is gone", Trump said with Conan at his side.
Neither the Justice Department nor a representative for McGahn was immediately available to comment.
The administration said it will appeal Jackson's ruling.
McGahn's lawyer, William Burck, previously wrote to Nadler that McGahn "remains obligated to maintain the status quo and respect the president's instructions".
The Justice Department will seek to put the ruling on hold in the meantime, department spokeswoman Kerri Kupec said.
White House press secretary Stephanie Grisham said in a statement that the court's ruling "contradicts longstanding legal precedent established by Administrations of both political parties".
Democrats subpoenaed McGahn in April of this year in connection with his testimony in the Mueller investigation.
The argument, in other words: information sought from McGahn doesn't involve national security matters, but that wouldn't be the case with Cooper's clients; we're talking about apples and oranges, so a judge needs to rule separately on Kupperman's case and Bolton's would-be case on whether they must testify.
The report would be transmitted to the House Judiciary Committee sometime next week, Schiff said, setting the stage for the next step in the impeachment process, the drafting of actual articles of impeachment. He also said the report would include a catalog of White House refusals to cooperate with the inquiry.