As Congressman James Comer gears up a probe into the inner workings of the White House during the Biden administration, the House Oversight Committee chairman is demanding interviews with Mr. Biden’s closest aides. One of those longtime loyalists to whom Mr. Comer wishes to speak recently went on the record about Mr. Biden’s diminished capacities. 

In a series of letters to several of Mr. Biden’s most senior aides, Mr. Comer says it is imperative that Congress get to the bottom of how the White House operated, and governed, during the final years of the administration when, according to a best-selling book by two establishment journalists, the president was exhibiting a significant decline in mental acuity. Mr. Comer and his GOP colleagues on the committee want to know what the staffers knew, and when they knew it. 

“The cover-up of President Biden’s mental decline is one of the greatest scandals in our nation’s history. These five former senior advisors were eyewitnesses to President Biden’s condition and operations within the Biden White House,” Mr. Comer said in a statement Wednesday. “They must appear before the House Oversight Committee and provide truthful answers about President Biden’s cognitive state and who was calling the shots.”

The five aides being called for closed-door interviews with the committee are two senior advisors, Mike Donilon and Anita Dunn; the deputy chief of staff, Bruce Reed; the presidential counselor, Steve Richetti, and the chief of staff, Ron Klain. 

The book by Jake Tapper and Alex Thompson, “Original Sin,” reports that this small circle of advisers — who had worked with Mr. Biden for decades — was referred to by other staff as “The Politburo,” and they tightly controlled access to the president. 

A senior editor at Reason magazine, Robby Soave, said on the political journalist Mark Halperin’s podcast last week that until he read the book, “I did not frankly realize how well concealed he was from other Democratic officials. I imagined a cover-up that must have involved many, many more people or a large number of staff. What I got from reading the book is, no, it was really just four people who saw it as their mission to make sure no one could see or talk to Biden for the entirety of his presidency, and also the well-documented proof that Joe Biden was ‘having good days and bad days,’ but some ‘bad days’ as far back as the end of his vice presidency.”

Mr. Comer specifically cites “Original Sin.”  In the book, one unnamed source tells the reporters: “Five people were running the country, and Joe Biden was at best a senior member of the board.” At the time Mr. Biden left the 2024 race, four of the five aides mentioned were still working in the White House. 

Mr. Klain left the administration in 2023, though he was acting as an outside advisor and helped run debate prep ahead of Mr. Biden’s fateful meeting with President Trump that eventually led to his dropping out. 

Mr. Comer has already subpoenaed three other Biden aides — Annie Tomasini, Anthony Bernal, and Ashley Williams — for their testimony about the president’s decline and the cover-up. “Original Sin,” as well as other press accounts, say these three aides were the gatekeepers in terms of anyone getting physically close or in earshot of the president. Mr. Bernal, who was the first lady’s senior adviser, had enormous power, according to published reports. 

The chairman also invited Mr. Biden’s physician, Kevin O’Connor, to appear for an interview. 

The committee chairman is asking all five of the top-level aides to sit for their interviews on June 16, and to confirm their intent to appear by June 11. If they cannot make June 16 work, then Mr. Comer says they must tell his staff what date works for them during that week of June 16. 

Mr. Donilon may be the Republicans’ great white whale in this fight, considering how long he has known the former president. He began his work for Mr. Biden in 1981 as a counselor — 40 years before he would sit behind the Resolute Desk. At a recent event at Harvard University, Mr. Donilon — whose brother, Tom, was President Obama’s national security adviser and who was paid $4 million by the Biden 2024 campaign — said he believed Mr. Biden would’ve won a second term had he stayed in the race. 

“During an event at Harvard University, you displayed your willingness to speak about the former president’s cognition but you reportedly ‘denounced claims that the president’s acuity and judgment declined,’” Mr. Comer writes in his letter to Mr. Donilon. “The scope of your responsibilities — both official and otherwise — and personal interactions within the Oval Office cannot go without investigation.”

Mr. Klain is also a key witness for the GOP, though he has already gone on the record about his concerns for Mr. Biden’s health. 

Speaking with journalist Chris Whipple, who wrote a best-selling book about the role of White House chief of staff, Mr. Klain said he knew Mr. Biden had lost a step when he helped him prepare for his debate with Mr. Trump last year. 

The Biden inner circle gathered at Camp David over the course of a week last June to help the president get ready. During those long sessions, Mr. Klain said he saw a change in the president. 

Mr. Klain is quoted in Mr. Whipple’s book “Uncharted” about the 2024 election as saying that Mr. Biden “didn’t really understand what his argument was on inflation” and that “he was just extremely exhausted” during debate prep. Mr. Klain later told Politico that he believed other White House staff had “isolated” Mr. Biden from the specifics of domestic policy.