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President Joe Biden's former chief of staff and a fixture of his re-election campaign, Ron Klain, will participated in a transcribed interview on Thursday before the Republican-led House Oversight Committee as part of the committee's investigation into the alleged cover-up of Biden’s cognitive decline.

In a letter requesting his appearance, House Oversight Chairman James Comer, R-Ky., quoted Klain as cutting Biden's debate prep short last year "due to the president’s fatigue and lack of familiarity with the subject matter," adding that Biden "didn’t really understand what his argument was on inflation," citing a POLITICO report from earlier this year. 

"If White House staff carried out a strategy lasting months or even years to hide the chief executive’s condition—or to perform his duties—Congress may need to consider a legislative response," Comer said, arguing the scope of Klain's responsibilities in his personal and professional capacities "cannot go without investigation."

Klain departed the White House in 2023 but served by Biden's side "for more than three decades," according to Comer's letter, citing reporting by The New York Times. Klain, who later returned to Biden's orbit to assist with debate prep, privately announced during Biden's disastrous debate performance: "We’re f---ed."

'MASSIVE CONSPIRACY': EX-DNC INSIDER SAYS PARTY WENT TO GREAT LENGTHS TO HIDE BIDEN'S MENTAL DECLINE

Former President Joe Biden smiles as his former chief of staff delivers a speech.

President Joe Biden hosts an official transition event for outgoing Chief of Staff Ron Klain. (HUM Images/ Universal Images Group via Getty Images)

"2024: How Trump Retook the White House and the Democrats Lost America," a new book released July 8 by journalists Josh Dawsey of The Wall Street Journal, Tyler Pager of The New York Times, and Isaac Arnsdor of The Washington Post, is one of several books pulling back the curtain on Biden's White House. 

PROBE INTO BIDEN'S ALLEGED MENTAL DECLINE COVER-UP DEEPENS WITH MORE FORMER WHITE HOUSE OFFICIALS TO TESTIFY

The authors described how, during the June 2024 debate, "Biden’s aides winced as the president started answering the first question about the economy and voters who felt they were worse off under his presidency." 

And backstage, as Biden stumbled over an answer that questionably ended with, "We finally beat Medicare," back in the holding room, Klain stood up and announced, "We’re f---ed," according to the authors. 

When reached for comment to confirm the book's allegation, Klain told Fox News Digital, "I have nothing to add."

Despite Klain expressing doubt internally, Klain continued to defend the president amid calls from donors and politicians for Biden to step down. 

President Biden at debate

President Biden speaks during his consequential debate performance in Atlanta, Georgia.  (Justin Sullivan/Getty Images)

On June 30, 2024, Klain reshared an X post that urged Americans to ignore the "news reports" with "anonymous sources about Dem donors calling for Biden to withdraw."

"We are the Democratic Party! These people don't get to decide to oust a pro-labor pro-people President," Klain said on July 4, 2024, in response to The New York Times reporting about the Democrats' pressure campaign against Biden. 

According to the book, after the debate, Klain called Jeff Zients, his successor as Biden's chief of staff, to say he was "disturbed that Biden was planning to spend the weekend at Camp David."

"We have an emergency," Klain told Zients, according to the book. "We have a crisis on Capitol Hill, and the crisis is going to accelerate."

But Zients insisted Biden was going to Camp David to be with his family, instead of Klain's plan to appease the progressive wing of the party with a bold second-term agenda. 

"I have no f---ing clue why he’s going to Camp David this weekend," Klain said, according to the authors. "He needs to be working the phones, day and night."

Ron Klain, Jeffrey Zients, and Joe Biden

Biden's former White House chief of staff, Ron Klain, is seen delivering remarks as Jeffrey Zients, right, looks on, in the East Room of the White House on Feb. 1, 2023, in Washington, D.C.  (Kevin Dietsch/Getty Images)

Even before the debate, when concerns about the first octogenarian president's ability to lead the country through a second term came to a boiling point, Klain had concerns, as portrayed in the book. 

Klain had overseen debate prep for every Democratic presidential candidate since 2004, according to the authors. Between Biden's cold, a shorter prep window than usual, and staffers privately expressing concern, debate prep at Camp David did not quite go as planned, the authors claimed.

"This is going to be really touch and go in Atlanta," Klain told other members of Biden's inner circle ahead of the debate, according to the book. 

But years prior, Klain shut down former President Barack Obama advisor David Axelrod for repeatedly calling Biden's age an issue. 

President Biden in Washington, D.C.

President Joe Biden listens during a visit to the D.C. Emergency Operations Center, on Tuesday, July 2, 2024, in Washington.  (AP/Evan Vucci)

"The presidency is a monstrously taxing job and the stark reality is the president would be closer to 90 than 80 at the end of a second term, and that would be a major issue," Axelrod told The New York Times.

But a source familiar with the Biden team's thinking told Fox News Digital that President Donald Trump and congressional Republicans are simply seeking "retribution" through a "partisan, coordinated effort."

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"It’s an attempt to smear and embarrass," the source said. "And their hope is for just one tiny inconsistency between witnesses to appear so that Trump’s DOJ [can] prosecute his political opponents and continue his campaign of revenge."

Klain, currently Airbnb's chief legal officer, did not immediately respond to Fox News Digital's request for comment ahead of his transcribed interview.