Former Pompeo aide testifies as Trump impeachment probe roars on

The Democrats' impeachment inquiry sprinted forward on Wednesday as a former State Department official arrived at the Capitol for what's expected to be another round of marathon testimony into the Trump administration's dealings with Ukraine.

Michael McKinley, a former top adviser to Secretary of State Mike PompeoMichael (Mike) Richard PompeoHillicon Valley: Treasury sanctions Russian group accused of targeting critical facilities | Appeals court rules Uber, Lyft must comply with labor laws | Biden: Countries that target US elections will 'pay a price' Treasury sanctions Russian group accused of targeting US critical facilities with destructive malware Trump announces opening of relations between Sudan and Israel MORE, is slated to discuss issues surrounding Pompeo's role in the administration's pressure campaign to get Ukrainian leaders to investigate one of Trump's leading political opponents, former Vice President Joe BidenJoe BidenFacebook, Twitter CEOs to testify before Senate Judiciary Committee on Nov. 17 Sanders hits back at Trump's attack on 'socialized medicine' Senate GOP to drop documentary series days before election hitting China, Democrats over coronavirus MORE.

That campaign, which resulted in the removal of Marie Yovanovitch from her post as U.S. ambassador to Ukraine in May, is at the center of the Democrats' hard-charging investigation into whether Trump sought to enlist foreign leaders to boost his reelection chances next year.

The inquiry has been fueled by allegations, lodged by a government whistleblower, that Trump had also threatened to withhold almost $400 million in U.S. aide to Ukraine if President Volodymyr Zelensky failed to comply with the administration's request.

Pompeo was reportedly on the July 25 phone call between Trump and Zelensky, when Trump asked his Ukrainian counterpart for "a favor" — a reference to opening the Bien investigation.

McKinley, a career diplomat who resigned last week, had been serving as U.S. ambassador to Brazil in 2018 when Pompeo brought him back to Washington to serve as an informal liaison between the agency brass and the department's career diplomats. His resignation was reportedly in protest of Pompeo's failure, as perceived by a growing number of State Department employees, to protect diplomats from the administration's Ukraine operations amid allegations that they were politically motivated.

With his testimony, McKinley becomes the fifth witness to appear before Congress since Speaker Nancy PelosiNancy PelosiOvernight Health Care: Following debate, Biden hammers Trump on coronavirus | Study: Universal mask-wearing could save 130,000 lives | Finger-pointing picks up in COVID-19 relief fight On The Money: Finger-pointing picks up in COVID-19 relief fight | Landlords, housing industry sue CDC to overturn eviction ban Finger-pointing picks up in COVID-19 relief fight MORE (D-Calif.) announced the launch of a formal impeachment inquiry just over three weeks ago. The closed-door deposition is being led by Rep. Adam SchiffAdam Bennett SchiffGreenwald slams Schiff over Biden emails on Fox Hillicon Valley: DOJ accuses Russian hackers of targeting 2018 Olympics, French elections | Federal commission issues recommendations for securing critical tech against Chinese threats | House Democrats slam FCC over 'blatant attempt to help' Trump Federal commission issues recommendations for securing critical tech against Chinese threats MORE (D-Calif.), chairman of the House Intelligence Committee, while two other panels — Oversight and Reform and Foreign Affairs — are also participating.

McKinley's appearance follows that of Kurt VolkerKurt 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 U.S. special envoy to Ukraine; Yovanovitch; Fiona Hill, who was formerly Trump's top Russia expert at the National Security Council; and George Kent, a deputy secretary of State, who testified for roughly 10 hours on Tuesday.

Democrats have emerged from each of the lengthy depositions with claims that the witness corroborated the whistleblower's charges, lending fuel to the Democrats' impeachment inquiry and fanning sentiments that the drafting of impeachment articles are all but inevitable.

Republicans, meanwhile, have defended Trump by focusing on the process, accusing Democrats of conducting a "clown show" — not an impeachment investigation — since the testimony is all being conducted privately and Pelosi has refused to stage a floor vote to launch the impeachment inquiry formally.

Olivia Beavers contributed.