Democrats have set a blistering pace for their impeachment inquiry of Donald Trump with a lineup of depositions -- including the recently departed U.S. envoy to Ukraine -- stoking the president’s fury and feeding efforts to discredit the investigation.

Kurt Volker, who stepped down last week from his unpaid role representing American interests in Ukraine, will give a closed-door deposition Thursday to the three House committees looking into Trump’s pressure on a foreign power to investigate a political rival, Joe Biden.

Lawmakers want to talk to Volker and others mentioned in the whistle-blower complaint of Trump’s July 25 call with Ukrainian President Volodymyr Zelenskiy.

As the committees delve into the allegations, Trump has made it clear that the impeachment process will be met not only with broad stonewalling, but also with fierce accusations leveled at those leading it, especially Intelligence Chairman Adam Schiff, a California Democrat.

Trump on Wednesday used Twitter and a press conference to denounce his political enemies, possibly setting the tone for the rest of this year -- as well as the 2020 presidential election.

Trump’s can call on reinforcements, most crucially administration officials like Secretary of State Michael Pompeo -- responsible for responding to the congressional subpoenas, even though he has been named in the allegations.

The president also has a cohort of surrogates and conservative media personalities echoing and fueling his outrage, most notably Rudy Giuliani, his personal lawyer. Giuliani says he has made it his personal mission to bring attention to what he calls shady dealings by Biden and his son Hunter in Ukraine.

No Pre-Conditions

These factors have amplified the drama and seeped into material collected as part of the impeachment inquiry, including documents disclosed during a closed-door Capitol Hill briefing on Wednesday from State Department Inspector General Steve Linick.

Pompeo has sought to limit his staff’s cooperation, accusing Democrats of trying to “bully” State Department officials.

Volker, however, agreed to sit for a deposition without a subpoena or any pre-conditions, according to a person familiar with the matter. His deposition will be the first of five former and current State Department officials called by the House committees on intelligence, foreign affairs and oversight that are leading the probe.

That includes Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled by Trump.

The government official who initially evaluated the whistle-blower’s complaint, Intelligence Community Inspector General Michael Atkinson, will testify behind closed doors on Friday.

House Republicans have followed Trump’s in lead questioning Schiff’s fitness to lead the inquiry, especially after revelations on Wednesday that the whistle-blower consulted with Intelligence Committee staff to ensure that the complaint would reach the relevant House panels. Trump repeated his call for Schiff to resign and alleged without evidence that Schiff had helped the unnamed intelligence official write the complaint.

Kevin McCarthy, the House Republican leader, said Democrats have “have rigged this process from the start.”

Schiff was already facing heavy criticism from Republicans for delivering a parody version of Trump’s phone call with Zelenskiy during a committee meeting following the release of White House records of the call. The president has repeatedly said his actions were treasonous.

“We don’t call him shifty Schiff for nothing,” Trump said. “He’s a shifty dishonest guy.”

Democrats on Wednesday denied Trump’s charge that Schiff helped write the whistle-blower’s complaint, saying committee staff simply advised the person to contact the inspector general and seek legal counsel. Schiff’s office said that at no point did the committee “review or receive the complaint in advance.”

“The whistle-blower should be commended for acting appropriately and lawfully throughout every step of the process,” Schiff spokesman Patrick Boland said in a statement.

Mark Zaid, an lawyer for the whistle-blower, said proper laws and processes had been followed.

“I can unequivocally state that neither any member of the legal team nor the whistle-blower has ever met or spoken with Congressman Schiff about this matter,” Zaid said.

The committee staffer who spoke with the whistle-blower did not reveal his or her identity to Schiff but did share elements of the complaint, the New York Times reported, helping to explain why Schiff knew to press the Trump administration to release details from the report.

The White House so far has not denied specific elements of the whistle-blower’s account, with Trump instead complaining that it unfairly portrays his request for an investigation into his political rivals in a sinister light.

‘Debunked Conspiracy Theories’

The files that Linick showed to members of Congress included emails from diplomats cautioning against unfounded reports that the U.S. Embassy in Kyiv was a hotbed of Democratic support.

The documents, which were obtained by Bloomberg News, also contain notes that purport to be from interviews that took place at Giuliani’s office.

Giuliani told CNN on Wednesday evening that some of the documents had originated with him.

The documents comprise a disjointed collection of material, from an envelope sent to Pompeo marked with “the White House” as the sender’s address. Inside there are divider sheets marked with a Trump Hotels label.

“The documents provided by the Inspector General included a package of disinformation, debunked conspiracy theories, and baseless allegations” Schiff, along with Oversight Chairman Elijah Cummings and Foreign Affairs Chairman Eliot Engel, said in a statement on Wednesday night. “These documents also reinforce concern that the President and his allies sought to use the machinery of the State Department to further the President’s personal political interests.”

Giuliani told CNN he provided some of the documents to the State Department.

Volker’s testimony could strike a different tone Thursday as he offers his version of how he tried to steer the unconventional connections between Trump associates and foreign leaders into official diplomatic channels.

Volker, who previously served as the U.S. ambassador to NATO, was initially asked by Secretary of State Rex Tillerson to help find a way to ease Ukraine’s conflict with Russia.

But he soon became embroiled in Giuliani’s investigation into the Bidens, and the whistle-blower complaint says he sought to “contain the damage” caused by Giuliani. Volker visited Kyiv, the Ukrainian capital, the day after Trump’s July 25 call with Zelenskiy, and he provided advice about how to “navigate” the American president’s request for an investigation of the Bidens.

According to a person familiar with the matter, Volker initially sought to reverse Trump and Giuliani’s impression that the newly elected Zelenskiy was an unreliable ally. Volker had been among those who argued for a call between Trump and Zelenskiy as part of broader State Department efforts to arrange a state visit for the Ukrainian leader.

Volker is the executive director of the McCain Institute for International Leadership at Arizona State University. The institute, named for the late Senator John McCain of Arizona, is based in Washington.

--With assistance from Nick Wadhams.