A former Trump adviser filed a lawsuit that could be 'one of the most consequential separation of powers cases' in history

Which branch trumps the other?

Charles Kupperman, President Trump's former deputy national security adviser under former National Security Adviser John Bolton, filed a lawsuit Friday asking a federal judge to rule whether he must testify in the congressional impeachment inquiry. House Democrats subpoenaed Kupperman, but the White House instructed him not appear before Congress, arguing he — along with Trump's other close advisers — is immune from testifying. And Kupperman really doesn't know what to make of it.

"Dr. Kupperman cannot satisfy the competing and irreconcilable demands of both the Legislative and Executive Branches, and there is no controlling judicial authority definitively establishing which Branch's command should prevail," Kupperman's lawyer, Charles Cooper, said in a statement.

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The Washington Post notes the case could become a major test of what has emerged as a consistent constitutional dispute during the Trump presidency, since it could eventually set a precedent as to whether the White House can really prevent Trump's advisers from testifying before Congress. "If this case is ultimately decided by the Supreme Court, it will be one of the most consequential separation of powers cases in American constitutional history — however it is decided," Judge J. Michael Luttig told the Post.

Kupperman is expected to testify Monday. He was on the July phone call between Trump and Ukrainian President Volodymyr Zelensky, making him the first witness with firsthand knowledge of the call to testify in the impeachment inquiry, The Wall Street Journal reports. Read more at The Wall Street Journal and The Washington Post.