Former White House counsel Donald McGahn, a key figure with firsthand knowledge of President Donald Trump's alleged efforts to short-circuit the Mueller investigation, must testify before Congress, a federal judge ruled Monday.

The ruling, which the Justice Department said it will appeal, affirms Congress' role as a check on executive power. If upheld, it could open the door for testimony by some of the president's closest aides, including former national security adviser John Bolton and acting White House Chief of Staff Mick Mulvaney.

U.S. District Judge Ketanji Brown Jackson rejected the White House's claims of absolute immunity, saying the president "does not have the power" to prevent his aides from responding to congressional subpoenas.

"Today, this Court adds that this conclusion is inescapable precisely because compulsory appearance by dint of a subpoena is a legal construct, not a political one, and per the Constitution, no one is above the law," Jackson wrote in a lengthy ruling issued late Monday.

"Stated simply, the primary takeaway from the past 250 years of recorded American history is that Presidents are not kings," Jackson wrote. "This means they do not have subjects, bound by loyalty or blood, whose destiny they are entitled to control."

McGahn, however, can invoke executive privilege "where appropriate," Jackson said, citing a previous court ruling that concluded former President George W. Bush's White House counsel must testify before Congress. The judge added it is "widely accepted" that the president can assert executive privilege to protect potentially sensitive topics.