Rep. Matt Gaetz, R-Fla., an ardent supporter of President Trump, got the boot on Monday when he tried to sit in on the testimony of a former top National Security Council expert on Russia who was appearing on Capitol Hill as part of the House impeachment inquiry into the president.

Gaetz, who sits on the House Judiciary Committee, attempted to attend the testimony of Fiona Hill, a former deputy assistant to the president, but was told that because he was not a member of the House Intelligence Committee that he had to leave. The House Intelligence, Oversight and Foreign Affairs committees are conducting the impeachment inquiry into Trump.

A frustrated Gaetz aired his disappointment to reporters after being told he was not allowed to sit in on the hearing, venting his anger over what he says are “selective leaks” by House Intelligence Committee Chairman Adam Schiff, D-Calif., and questioning why he was not allowed to be present during Hill’s testimony. Gaetz added that the chairman of the House Judiciary Committee, Rep. Jerry Nadler, D-N.Y., was involved in the impeachment inquiry.

“It’s not like I’m on agriculture,” Gaetz said. “What are the Democrats so afraid of?”

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Gaetz followed up his comments with a tweet calling the impeachment inquiry a kangaroo court and using one of Trump’s favorite nicknames for the intelligence committee chairman, “Shifty Schiff.”

"Judiciary Chairman [Jerry Nadler] claimed to have begun the impeachment inquiry weeks ago,” Gaetz tweeted. “Now, his own Judiciary members aren’t even allowed to participate in it. And yes - my constituents want me actively involved in stopping the #KangarooCourtCoup run by Shifty Schiff."

Other Republicans closely aligned with Trump continued on Monday to complain about Schiff and his handling of the impeachment inquiry – with Rep. Jim Jordan, R-Ohio, also lambasting the California Democrat for excluding some congressional Republicans from the testimonies and for leaking “cherry-picked” information from the closed-door hearings to the press.

“She was going to come voluntarily but he's going to subpoena her I believe so he can ask certain questions and again keep those secret except for the certain things that he wants to leak, the cherry-picked information to the American people,” Jordan said of Schiff before Hill’s testimony.

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Lee Wolosky, Hill’s attorney, tweeted on Monday that the former deputy assistant to the president had received a congressional subpoena.

“The tragedy here and the crime here is that the American people don't get to see what's going on in these sessions,” Jordan said.

Hill’s testimony comes ahead of a planned Thursday appearance by Gordon Sondland, Trump's hand-picked ambassador to the European Union, and follows the revelation of a cache of text messages from top envoys that provide a vivid account of their work acting as intermediaries around the time Trump urged Ukraine's new president, Volodymr Zelenskiy, to start investigations into a company linked to the family of a chief Democratic presidential rival, Joe Biden.

Sondland is set to tell lawmakers that he did understand the administration was offering Zelenskiy a White House visit in exchange for a public statement committing to investigations Trump wanted, according to the person, who demanded anonymity to discuss remarks not yet given.

But Sondland will say he did not know the company being talked about for an investigation, Burisma, was tied to Joe Biden's son, Hunter Biden, the person said. Sondland understood the discussions about combating corruption to be part of a much broader and publicized Trump administration push that was widely shared, the person said.

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One witness who may not be called before Congress is the still anonymous government whistleblower who touched off the impeachment inquiry. Top Democrats say testimony and evidence coming in from other witnesses, and even the president himself, are backing up the whistleblower's account of what transpired during Trump's July 25 phone call with Zelenskiy.

Lawmakers have also grown deeply concerned about protecting the person from Trump's threats over the matter and may not wish to risk exposing the whistleblower's identity.

Schiff said Sunday, "We don't need the whistleblower, who wasn't on the call, to tell us what took place during the call. We have the best evidence of that." He added it "may not be necessary" to reveal the whistleblower's identity as the House gathers evidence.

"Our primary interest right now is making sure that that person is protected," Schiff said.

Fox News’ Chad Pergram and The Associated Press contributed to this report.