WASHINGTON – When Kurt Volker took an unpaid, part-time position in the Trump administration as a special envoy for Ukraine, he probably didn't imagine it would land him in the witness chair of an impeachment inquiry.

But Volker – a former foreign service officer and long-time Europe expert – will now be the first official interviewed by House Democrats, as they investigate allegations that President Trump, during a July 25 call with Ukrainian President Volodymyr Zelensky, used the power of his office to seek foreign interference in the 2020 presidential election.

Volker, who resigned Friday from his special envoy post, is scheduled to be deposed Thursday during a closed hearing on Capitol Hill. He played a central role in connecting Trump's personal attorney, Rudy Giuliani, with Ukrainian officials – a step critics say is highly inappropriate. And he was named in the explosive whistleblower complaint that sparked the impeachment proceedings.

Volker resigned less than 24 hours after Giuliani posted a private text message from the special envoy – in which Volker offered to set up a meeting with a top adviser to Zelensky. Giuliani was trying to get Ukrainian officials to investigate former Vice President Joe Biden,a political rival who is seeking to unseat Trump in 2020.

“As discussed, connecting you here with Andrey Yermak, who is very close to President Zelensky,” the July 19 text message reads. “I suggest we schedule a call together on Monday – maybe 10 am or 11 am Washington time?”

Democrats slammed Volker's text message as evidence of "highly illegal coordination" between the Trump campaign and the State Department. On Thursday, they will get their first opportunity to grill Volker over his dealings with Giuliani and Ukrainian officials.

"He’ll get some hostile questions," said John Herbst, a former U.S. ambassador to Ukraine who is now director of the Atlantic Council’s Eurasia Center. But Herbst, who has known Volker for over a decade, says he will be able to offer lawmakers a persuasive defense for his actions.

"He’s a smart guy and a cool guy, and I think he’ll have reasonable answers," Herbst said.

Herbst said Volker was focused on furthering U.S. foreign policy goals – namely helping Ukraine secure the military assistance it needs to counter Russian aggression. Trump temporarily froze U.S. aid to Ukraine in mid-July, a move that prompted bipartisan outrage as the eastern European country struggled to fight Moscow's attacks on its fledgling democracy.

"He had a job to pursue American interests in Ukraine, to help Ukraine deal with the Kremlin aggression," Herbst said of Volker. He tried to do that by reaching out to Giuliani, while also trying to maintain bipartisan support in Congress for Ukraine.

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Volker is a "very smart, balanced, creative professional" who is willing to take risks if he believes it will help achieve America's interests abroad, he said. "Unfortunately this did not turn out well, but it was not an unreasonable thing for him to attempt."

According to the whistleblower complaint, Volker and another U.S. ambassador, Gordon Sondland, had met with Giuliani to try to "contain the damage" his efforts were having on U.S. national security. The whistleblower said Volker and Sondland also met with Ukrainian officials to help them navigate the "differing messages" they were getting through official U.S. government channels and Giuliani's private outreach.

Kelly Magsamen, a former national security and defense official in the Obama administration, said Volker “has a record of being a very serious guy” and is seen as part of the GOP establishment’s foreign policy crowd.

“He’s not necessarily what I would consider a Trumpian person,” said Magsamen, now a national security and international policy expert at the liberal Center for American Progress. “That said, I do think he has a lot of things to answer for possibly tomorrow.”

Magsamen said Volker will likely be asked a raft of sensitive questions: What did he know about the July 25 Trump-Zelensky phone call, was he asked to assist Giuliani’s campaign to get damaging information on the Bidens, were Giuliani’s efforts considered part of U.S. foreign policy or a “rogue” operation, and did Trump withhold U.S. security assistance to Ukraine as leverage over Zelensky.

“I think Ambassador Volker is probably eager to clarify his role in these issues with the Hill,” she said, adding that he has a “strong interest in salvaging his reputation.”

It's not yet clear if Volker will be questioned by lawmakers or by staff lawyers on the House committees leading the probe, or both.

Volker is moving ahead with Thursday's deposition, even as Secretary of State Mike Pompeo has delayed other State Department officials from testifying. Volker has not responded to multiple emails seeking comment. But because he resigned from his government post, he may have more freedom to talk to House Democrats than others who still work for the Trump administration.

Giuliani has for months tried to push Zelensky's government to dig up damaging information on Biden and his son, Hunter Biden, who served on the board of directors of a Ukrainian energy company. That company had come under scrutiny by Ukraine's former chief prosecutor for possible corruption. But Ukrainian officials have said there's no evidence of wrongdoing by either Joe Biden or his son.

Volker was named as Trump's special envoy in July 2017, by then-Secretary of State Rex Tillerson. He came to the post with a stellar resume: a former U.S. ambassador to NATO in the George W. Bush administration and former adviser to Sen. John McCain, R-Ariz., a longtime Russia critic. He took the envoy job on a volunteer basis, while continuing to serve as executive director of the McCain Institute for International Leadership at Arizona State University.

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Volker's mission as special envoy was to help enforce a political settlement to the conflict between Russia and Ukraine. Russia annexed the Crimean peninsula in 2014, and Ukraine continues to fight Russian separatist attacks in the eastern part of the country. Experts say it's vital for the U.S. to help Ukraine stave off Russian aggression, particularly as Vladimir Putin looks to rebuild the Soviet empire.

Sen. Chris Murphy, a Connecticut Democrat who sits on the Senate Foreign Relations Committee, last week tweeted out praise for Volker as having "a well deserved reputation for fairness, toughness and integrity, which is why I was so disappointed to see him caught up in this mess."

Volker's appearance is the first of what Democrats hope will be a parade of officials who have knowledge either of the administration's attempts to pressure the Ukrainian government for help going after the Bidens or attempts by the White House to "lock down" information related to Trump's July 25 call to Zelensky as the whistleblower complaint alleges.

House Intelligence Chairman Adam Schiff, D-Calif., told reporters Wednesday that Volker's deposition before the committee will be followed Friday by an appearance from Michael K. Atkinson, the inspector general of the intelligence community, whose office handled the whistleblower complaint.

"The last time that the inspector general testified, we did not have the complaint. We now do. And we certainly intend to ask (him) about the efforts that were made to corroborate that compliant which we now know that the inspector general found credible and urgent," Schiff said.

Next week, the committee has scheduled a deposition of Marie Yovanovitch, the former U.S. ambassador to Ukraine who was abruptly recalled from her post in May. And Schiff said the committee is "in discussion" with other State Department witnesses to secure their depositions.

"We are proceeding deliberately but at the same time we feel a sense of urgency here that this work needs to get done and it needs to get done in a responsible period of time," he said.