A former top aide to President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on European and Russian affairs has arrived on Capitol Hill to testify before Congress, where she is expected to detail a “shadow foreign policy” pursued by Trump's personal lawyer Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE and U.S. Ambassador to the European Union Gordon Sondland.

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Fiona Hill, a former special assistant to the president, on Monday is set to testify that Sondland and Giuliani went around the National Security Council and official White House protocol to speak directly with Trump about Ukraine, NBC reported last week.

Hill’s attorney, Lee Wolosky, confirmed that his client had received a subpoena and intended to answer questioning from members beginning at 10 a.m. She is the first former White House official to agree to cooperate with the House’s impeachment inquiry.

Our client, Dr. Fiona Hill, former Dep Asst to Pres, received a congressional subpoena, will comply and answer questions from Members this morning starting at 10 AM. — Lee Wolosky (@LeeWolosky) October 14, 2019

Hill will also reportedly testify that she strenuously objected to the removal of former ambassador to Ukraine Marie Yovanovitch, but that her input was disregarded, according to The New York Times.

Hill entered the secured meeting room in the Capitol basement at roughly 9 a.m., an hour before the deposition was scheduled to begin. She declined reporters' requests for comment.

Republican leaders were more talkative ahead of the hearing.

Reps. Lee Zeldin Lee ZeldinDCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program Overnight Defense: House panel probes Pompeo's convention speech | UN council rejects US demand to restore Iran sanctions | Court rules against Pentagon policy slowing expedited citizenship The Hill's 12:30 Report: Republicans conduct in-person convention roll call MORE (R-N.Y.), Scott Perry Scott Gordon PerryOn The Trail: How Nancy Pelosi could improbably become president GOP lawmaker: Systemic racism doesn't exist and there's 'more to the story' of Floyd's death We're united in an effort to end the FDA's dog testing mandate MORE (R-Pa.), and Jim Jordan James (Jim) Daniel JordanHouse panel pulls Powell into partisan battles over pandemic Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election House passes resolution condemning anti-Asian discrimination relating to coronavirus MORE (Ohio), senior Republican on the House Oversight and Reform Committee, spoke with reporters on their way into the room, accusing Intelligence Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) of conducting an underhanded process by releasing only selected portions of testimony from previous witnesses.

"The tragedy here — the crime here — is that the American people don't get to see what's going on in these sessions," Jordan said of the closed-door session.

The Republicans, in particular, are keen on making public the transcript of the Oct. 3 testimony of Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE, the administration's former special envoy to Ukraine, who appeared before the committees on Oct. 3.

"He said there was no quid pro quo whatsoever, this process that played out is the typical diplomatic process that plays out when the hard-earned tax dollars of the American people are going to foreign governments," Jordan said.

Trump and his Republican allies are demanding a vote on the House floor to launch an impeachment inquiry formally — a step that preceded the last two impeachment processes, targeting Presidents Nixon and Clinton. They're protesting that, as the minority party, they're denied certain powers, such as the right to call witnesses themselves.

"If the House Democrats want to pursue impeachment, they should put their money where their mouth is and have a vote," said Zeldin.

Asked which witnesses Republicans would like to hear from, Zeldin did not name names, lamenting that many of the figures cited in documents and testimony are anonymous.

"We just want facts," he said.

Unlike Volker, Hill will reportedly not present any documents or communications, as she left them behind when she left the White House, according to the Times.

Sondland himself is set to testify Thursday and will reportedly say that a text message in which he said there was “no quid pro quo” conditioning military aid to Ukraine on an investigation of former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE’s family was dictated by Trump himself and that Sondland was uncertain of its accuracy.

The July 25 call between Trump and Ukrainian President Volodymyr Zelensky is at the heart of a whistleblower complaint that prompted the House to announce an impeachment inquiry into Trump last month.

Updated at 10:35 a.m.