WASHINGTON — A key witness in the impeachment investigation filed a lawsuit Friday asking a federal judge to rule on whether he can testify, a move that raises new doubts about whether President Trump’s closest aides, like the former national security adviser, John R. Bolton, will be able to cooperate with the inquiry.

The article states the following;

House Democrats had subpoenaed the witness, Charles M. Kupperman, who served as Mr. Trump’s deputy national security adviser, to testify on Monday. But in an effort to stop Mr. Kupperman from doing so, the White House said on Friday that the president had invoked “constitutional immunity,” leaving Mr. Kupperman uncertain about what to do.
“Plaintiff obviously cannot satisfy the competing demands of both the legislative and executive branches, and he is aware of no controlling judicial authority definitively establishing which branch’s command should prevail,” the suit said.

The implications of the suit, filed in federal court in Washington, extend beyond Mr. Kupperman. His lawyer, Charles J. Cooper, also represents Mr. Bolton and is likely to address congressional requests for his testimony in a similar fashion. House Democrats have had discussions with Mr. Cooper in recent days about Mr. Bolton testifying but have not subpoenaed him.

Democrats believe that Mr. Kupperman and Mr. Bolton could be significant witnesses for their investigation. Unlike several of the administration officials who have already testified, they were both close advisers of Mr. Trump, dealt directly with him on Ukraine policy and could testify about what Mr. Trump said behind closed doors.

Mr. Trump and the White House have attacked many of the career State Department officials who have appeared before investigators on Capitol Hill, calling them “unelected bureaucrats.”

But Mr. Kupperman and Mr. Bolton, longtime Republicans, worked directly for Mr. Trump. Mr. Bolton, in particular, is seen as potentially having greater sway with Republicans and independents because of his hawkish views, which he conveyed regularly on Fox News before joining the administration.

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