Former U.S. Attorney Guy Lewis told "America's Newsroom" on Tuesday that the Trump administration is “drawing a line in the sand" by blocking the U.S. ambassador to the European Union from testifying in a House impeachment deposition on the Ukraine controversy.

“They’re arguing to the American people that process matters; procedure matters,” Lewis said.

Trump dismissed the deposition scheduled on Tuesday as a "kangaroo court" as he defended the State Department's decision to block Gordon Sondland from appearing at the closed-door session on Capitol Hill.

TRUMP ADMINISTRATION BLOCKS AMBASSADOR FROM TESTIFYING IN HOUSE IMPEACHMENT DEPOSITION

The decision triggered a fiery new clash with House Democrats amid their impeachment inquiry.

But the move, first reported by The New York Times, prompted new warnings from Democrats that such acts will be seen as an obstruction of their investigation into the controversy surrounding Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky.

HOUSE DEMS SUBPOENA PENTAGON, WHITE HOUSE BUDGET OFFICE IN IMPEACHMENT INQUIRY

In the call, which was flagged as part of a whistleblower complaint, Trump urged Zelensky to look into possible interference by Ukraine in the 2016 election as well as former Vice President Joe Biden and his son Hunter’s conduct in the country.

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“If the Democrats are going to prevent us from even knowing who the whistleblower is, being able to question the whistleblower, putting him behind some kind of curtain like the Wizard of Oz, then we’re not going to participate in the process, it’s unfair. That’s their message to the jury,” Lewis said.

“We’re trying to impeach the president of the United States, overturn an election, so I think they [Democrats] need to go back to the drawing board and proceed a little more orderly."

Fox News’ Adam Shaw contributed to this report.