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(CNN) —  

Former US Special Envoy for Ukraine Kurt Volker is set to appear Thursday before three congressional committees – the first official to testify on explosive whistleblower charges that President Donald Trump tried to pressure Ukraine to investigate his Democratic rival Joe Biden, an effort the White House then worked to cover up.

A longtime Republican foreign policy expert who was seen in the White House as not fully “on the Trump train,” Volker is now at the center of the impeachment inquiry examining allegations Trump deployed the machinery of state in a vendetta to target political rivals.

Volker’s testimony, analysts say, could be damaging to the President and his allies.

Volker is seen as “a well-respected straight shooter who is likely to testify in ways that will damage other Trump allies,” Ian Bremmer, president of the Eurasia Group, wrote in an emailed analysis. “It’s possible others, including key administration members like Secretary of State Mike Pompeo and Attorney General William Barr would also be forced out.”

Volker is also one of the State Department witnesses whose depositions to Congress Secretary of State Michael Pompeo tried to block, setting off a tussle with lawmakers who say they will interpret any effort to prevent the officials’ appearances as obstruction.

Where will Volker appear and why?

Volker is now set to testify before the Intelligence, Oversight and Reform and Foreign Affairs committees behind closed doors.

Dozens of pages of documents were delivered to those House committees Wednesday on behalf of Volker ahead of his Thursday deposition, two sources familiar tell CNN.

What remains unclear is if the documents were cleared by the State Department to be handed over, or if they are considered Volker’s personal documents. The State Department did not respond to CNN’s request for comment.

The announcement about his appearance came just hours before Volker resigned from his State Department position Friday, following release of the whistleblower complaint and a White House transcript of Trump’s July 25 call with Ukrainian President Volodymyr Zelensky.

In that call, Zelensky made clear that Ukraine, vulnerable and weak in the face of Russian aggression, wanted to buy American anti-tank missiles. Trump answered: “I would like you to do us a favor though.”

Trump pushed Zelensky to work with his personal lawyer Rudy Giuliani and Barr to investigate former Vice President Joe Biden and his son, Hunter, who had business dealings in Ukraine. There is no evidence of wrongdoing by Joe or Hunter Biden.

Trump also urged Zelensky to find a Democratic National Committee computer server that the US intelligence community says was hacked by Russian intelligence during the 2016 election campaign. There is no evidence Ukraine had any involvement in the hack.

The whistleblower complaint said that a day after Trump’s call, Volker and the US ambassador to the European Union, Gordon Sondland, met with Zelensky and provided advice about how to “navigate” Trump’s demands.

The State Department said this summer that Volker put Ukrainian Presidential Adviser Andriy Yermak in contact with Giuliani, at Yermak’s request.

A source familiar with Volker’s thinking said he was trying to get the Biden-Giuliani issue off the table, given that it was a political effort and not one that he was meant to support in his State Department role.

Volker did not want the controversy to occupy the center of bilateral conversations either, the source said, and hoped that in connecting Giuliani with Yermak, it would lead to a conversation in which the Ukrainians would agree to look into the matter and it would eventually fizzle out in two or three years.

Giuliani, meanwhile, has repeatedly claimed that these meetings were arranged at the State Department’s request, pointing to Volker as the official who reached out to arrange them and showing text exchanges on television to back up that claim.