Radio host Mark Levin on Monday slammed a D.C. federal judge's ruling that former White House counsel Don McGahn must appear before Congress pursuant to a subpoena issued earlier this year.

"She's a radical leftist and this is truly outrageous," Levin said on his radio show.

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If McGahn wanted to assert executive privilege to avoid testifying as part of the House impeachment inquiry, U.S. District Court Judge Ketanji Brown Jackson ruled, he would need to appear before Congress and do it himself, likely on a question-by-question basis.

"Now, this is what you need to understand, a president must be able to have legal advice. Must be able to have legal advice without Congress interfering," Levin said. "Whether it's an impeachment proceeding or any other proceeding. Otherwise, there's no balance of power because the House of Representatives, unless there's a criminal investigation, is not subject to any subpoenas."

"If a president can't turn to a lawyer for legal advice, then it's a disaster," Levin added, saying the ruling would be appealed.

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The host of "Life, Liberty & Levin" called the judge, who was appointed by former President Obama, a "disgrace" and noted the signifcance of the ruling.

"So, what she's doing is she's tilting the balance of power far away from the president to the Congress. Changing the structure of our government," Levin said. "And the ability of the chief executive to function, to function. There is a circle of advice that a president gets in the inner circle that should be unmolested by these subpoenas."

Fox News' Gregg Re contributed to this report.