A senior State Department official arrived in the Capitol on Wednesday to testify in the Democrats' impeachment investigation, bucking a White House that has vowed to offer no cooperation in the fast-moving inquiry.

David Hale, the under secretary of State for political affairs, is expected to provide insights into the campaign by 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 and some of his allies to remove Marie Yovanovitch from her post as U.S. ambassador to Ukraine — a move that infuriated many department veterans, who have expressed concerns that the agency's top brass did too little to protect a career diplomat from White House interference.

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Hale, a career dilplomat, arrived in the Capitol shortly before the start of Wednesday’s 9 a.m. deposition in the Capitol basement, where the three House committees leading the impeachment investigation — Intelligence, Oversight and Reform, and Foreign Affairs — have been conducting their closed-door interviews.

Democrats leading the inquiry have invited three other administration officials to testify on Wednesday: Energy Secretary Rick Perry Rick PerryEnergy secretary questions consensus that humans cause climate change OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project MORE; State Department counselor Ulrich Brechbuhl, a top adviser to Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE; and Russell Vought, acting director of the White House Office of Management and Budget.

None of the other three are expected to testify.

Brechbuhl boarded a plane heading to Germany with Pompeo earlier Wednesday. Last week, an Energy Department spokeswoman said Perry would not cooperate with the inquiry. And Vought had previously indicated that he did not plan to comply with the House inquiry with a tweet.

And Hale is the first witness to give a deposition this week after a series of witnesses refused to testify, offering various reasons.

Hale’s testimony comes as Democrats are shifting their impeachment investigation — which has been conducted in private for the six weeks since it launched — into the public realm.

As part of that transition, Democrats this week have released the verbatim transcripts of the testimony from some of the previous witnesses. That list includes Yovanovitch; Michael McKinley, a former top advisor to Pompeo; 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, former special envoy to Ukraine; and Gordon Sondland, a Trump ally and U.S. ambassador to the European Union.

Most of those witnesses have painted a damning picture of a shadow foreign policy in Ukraine — led by 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, Trump’s personal lawyer — to pressure Ukrainian leaders to open investigations into Trump’s political opponents. A government whistleblower has taken the narrative a step further, alleging that Trump threatened to withhold U.S. military aid to Ukraine unless the country’s leaders launched those investigations — the accusation at the root of the Democrats’ impeachment inquiry.

Sondland, in his initial deposition on Oct. 17, had denied such a quid pro quo had occurred. On Monday, however, he revised his testimony to acknowledge that he, himself, had delivered the message to Ukrainian leaders that the military aid was “likely” contingent on Ukraine’s willingness to open the investigations Trump had sought.

Democrats have pointed to the revision as new evidence that Trump had abused his office in potentially impeachable ways. The president’s GOP allies, though, have dismissed Sondland’s addendum as the opinion of one witness — and not reflective of Trump’s foreign policy in Ukraine.

They’re pointing to the account of Volker, who told lawmakers that the hold on military aid was “not significant” to U.S.-Ukrainian relations.

Rep. 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.) suggested Wednesday that Volker is the only truly reliable witness lawmakers have heard from so far. The multiple other witnesses claiming a quid pro quo, he said, were either biased or ill-informed.

“Some were leaving, some were about to leave, some had just come into the situation, some had preconceived notions regardless,” Perry said, as he and Rep. 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 (R-Ohio) headed into Hale’s deposition. “He seems to be the one honest broker in this whole thing.”

Updated at 9:57 a.m.