A key diplomat and donor 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 is scheduled to appear Thursday before three House committees in highly anticipated testimony that could shed more light on the Ukraine controversy roiling the nation’s capital.

Gordon Sondland, the U.S. ambassador to the European Union, was blocked from testifying last week by the Trump administration. The White House initially vowed not to cooperate with the House impeachment inquiry into allegations that the president abused his office by urging a foreign leader to deliver political dirt on 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. It’s unclear if the State Department will attempt to prevent his appearance this time. But Democrats have subpoenaed the wealthy hotel magnate, and his lawyers have indicated he intends to comply.

Sondland will be the sixth witness to testify in the Democrats’ investigation, and he could prove to be among the most controversial. Sondland, a former businessman who donated $1 million to the president’s inauguration committee, has emerged as a vocal Trump loyalist who quickly defended the president’s Ukraine dealings in the face of concern from other leading U.S. diplomats.

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Text messages turned over to Congress this month by 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, revealed that Volker and Sondland had sought to entice Ukrainian President Volodymyr Zelensky into launching an anticorruption investigation into Biden, a top 2020 presidential contender, and his son by dangling promises of a meeting with Trump if Zelensky agreed.

The text messages also reveal a conversation last month between Sondland, Volker and William Taylor, the current chargé d’affaires for Ukraine, in which Taylor seemed to express grave concerns that Trump was making the release of U.S. military aid to Ukraine contingent upon Zelensky’s decision to investigate the Bidens.

“As I said on the phone, I think it’s crazy to withhold assistance for help with a political campaign,” Taylor wrote in a Sept. 9 message.

Sondland replied, “Bill, I believe you are incorrect about President Trump’s intentions.”

“The president has been crystal clear no quid pro quo’s of any kind,” Sondland wrote. “The president is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign. I suggest we stop the back and forth by text.”

Sondland reportedly consulted with Trump before sending the text on the “no quid pro quo” claim.

The Wall Street Journal, however, reported this month that Sondland had told Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination MORE (R-Wis.) that the administration had dangled the military aid in return for the investigation — a report that’s sure to be of interest to Democrats questioning Sondland on Thursday.

“In a Wall Street Journal report, apparently he talked to Sen. Ron Johnson. And he said that, basically, the reason why Ukrainian aid was being held up was because the Ukrainians had not yet agreed to an investigation of the Bidens,” Rep. Raja Krishnamoorthi Subramanian (Raja) Raja KrishnamoorthiCDC causes new storm by pulling coronavirus guidance Democratic chairman says White House blocked Navarro from testifying Democrats urge CDC to update guidance to encourage colleges, universities go tobacco-free MORE (D-Ill.), a member of both the House Intelligence and Oversight and Reform committees, told CNN on Tuesday. “In other words, meddling in our 2020 elections.”

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“He knows a lot, and we’re hoping to learn more from him about the substance and veracity of those allegations in the whistleblower complaint,” Krishnamoorthi added.

Democrats are also focused on Sondland’s associations with 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, who was deeply involved in the pressure campaign on Ukraine. Part of that effort involved the removal of Marie Yovanovitch, the former U.S. ambassador to Ukraine, from her post in May — a move that infuriated State Department veterans.

“The most important thread that we’re teasing out of all of this is the fact that the president of the United States has been running a shadow foreign policy with Rudy Giuliani, who is a private citizen,” said Rep. Jackie Speier Karen (Jackie) Lorraine Jacqueline SpeierOvernight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies House to vote on 'I Am Vanessa Guillén' bill Overnight Defense: Trump's battle with Pentagon poses risks in November | Lawmakers launch Fort Hood probe | Military members can't opt out of tax deferral MORE (D-Calif.), a member of the Intelligence Committee. “He has no credentials, and it is confusing our allies around the world — and thrilling our adversaries.”

The president’s defenders have asserted Trump did not use the military aid for leverage, pointing to Sondland’s text message as further evidence. Many Republicans were disappointed last week when the State Department blocked Sondland’s testimony, which GOP lawmakers see as potentially exonerating the president of wrongdoing.

Still, as both sides are seizing on details to support their competing narratives, it is said that Sondland wants the opportunity to share his side of the story to set the record straight.

Sondland’s rescheduled appearance, however, comes amid increased scrutiny of his role in carrying out Ukraine policy, which does not fall under his umbrella as a diplomatic representative to the European trading bloc.

Fiona Hill, Trump’s former leading Russia expert, testified before House investigators earlier this week that Sondland was grouped with other Trump officials who were assisting Trump and Giuliani in their contacts with Ukraine.

And other experts have raised concerns that Giuliani, Sondland and others conducted a shadow diplomacy with Ukraine, one which falls outside of the normal channels.

Hill told House investigators that then-national security adviser John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE was so alarmed by what he heard about Trump’s contacts with Ukraine that he instructed her to notify the chief lawyer for the National Security Council about the efforts of Giuliani, Sondland and acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE, The New York Times reported this week.

“I am not part of whatever drug deal Sondland and Mulvaney are cooking up,” Bolton told Hill, according to her reported testimony.