GEOFF BENNETT: It was around this time last year that a single debate upended the Democratic presidential ticket and altered the course of American political history.

That night revealed a version of Joe Biden that, according to journalists Jake Tapper and Alex Thompson, those closest to the former president had gone to great lengths to shield from public view.

I spoke with Tapper and Thompson earlier today about the reporting and revelations detailed in their new book, "Original Sin."

Jake Tapper, Alex Thompson, welcome to the "News Hour."

JAKE TAPPER, Co-Author, "Original Sin: President Biden's Decline, Its Cover-Up, and His Disastrous Choice to Run Again": Great to be here.

GEOFF BENNETT: So the central thesis of "Original Sin" is that Biden's inner circle concealed his cognitive decline from the public.

And that you say, is a cover-up.

That could be a loaded phrase.

But why is it the right one, Jake?

JAKE TAPPER: Well, first of all, it was not just his inner circle.

It was also him and his family concealing this.

But I think it's a fair use of the term, not - - we're not alleging a criminal cover-up, but we are saying that the definition of cover-up, when you're hiding something bad from people, is unambiguously true, especially in 2023-2024, when they were cutting off access to President Biden from Cabinet secretaries, even members of the White House staff, Democratic officials, so as to show as little of this non-functioning Joe Biden, which was not 24/7 -- it was just on occasion -- as possible.

The Joe Biden we saw at the debate June 27, 2024, that's the non-functioning Joe Biden.

And the reason so many people were shocked, even though the world had been watching him age and having moments, is because they kept that non-functioning Joe Biden from the public as much as possible.

GEOFF BENNETT: Is the suggestion, though, that he was no longer able to make key decisions, that he was incapable of doing it?

ALEX THOMPSON, Co-Author, "Original Sin: President Biden's Decline, Its Cover-Up, and His Disastrous Choice to Run Again": I think what we have is that senior members of the Democratic Party, members of the Senate Cabinet, senior White House officials did have those concerns.

And, honestly, because they were being shielded from him, I think some of those concerns grow.

We had multiple Cabinet members tell us that, in 2024, if there was that proverbial 2:00 a.m. phone call crisis, that they were worried, that they didn't have full confidence that he could necessarily meet that moment.

GEOFF BENNETT: You likened former President Biden's senior aides to a politburo, suggesting centralized control, secrecy, a lack of transparency.

What exactly were they hiding and how far did it go?

ALEX THOMPSON: Well, to be clear, it was members of the administration's that called them the politburo.

And it was because of that incredible control.

And, basically, they controlled the most essential part of every single presidency, which was his time and access to him and which decisions were getting to him.

GEOFF BENNETT: Were there decisions, based on your reporting, that were effectively made by other people, consequential decisions?

ALEX THOMPSON: I mean, Cabinet members worried about that, essentially, especially by 2023-2024.

We had Cabinet members' access cut off and then eventually they would -- these senior people, senior at the politburo, would have them and come and brief them.

And there's one Cabinet member in particular, I still remember, said, yes, the president is technically making the final decision, but suspected that the politburo was framing the decisions in a certain way that it wasn't much of a decision.

GEOFF BENNETT: One of the questions that I have heard in connection to this book is, why is this all coming to light now when the American public was clear, I mean, this was documented in polling throughout the campaign, that there were real questions about Joe Biden's age?

And you have heard some people, mostly on the right, say all this book does is prove that the press was complicit, that the press had a role to play in this perceived, this alleged cover-up.

ALEX THOMPSON: I mean, if we knew all the things in the book, we would have definitely reported it before the election.

The problem was that, if you're doing reporting, you need sources to be honest.

And a lot of Democrats basically did not want to speak up because they feared it would hurt Joe Biden and then Kamala Harris and help Donald Trump.

We didn't start this book until November 6, the day after the election.

And, finally, people that we have been trying to reach out to for years started talking to us, people that had previously not been candid started becoming more candid.

And I think it also speaks to the fact that members of the Democratic Party, beyond just the politburo and the White House, they saw non-functioning Biden in the months before and they didn't speak out.

JAKE TAPPER: One of the things I hear -- and, look, there is valid criticisms of the news media, including me, including Alex, including all of us, in terms of how much we were able to uncover during the Biden presidency and the skepticism that we showed or did not show.

But, that said, there is a difference between showing a clip of Biden tripping or saying something uncomfortable or awkward and commenting on it, observing it, making fun of it, whatever, there's a difference between that and a deeply researched, deeply reported 85,000-word book that investigates what was really going on behind the scenes.

I have seen people say, oh, we suspected that all along.

OK, we proved it.

It's different.

I'm not saying that there is anything wrong with what they did.

In fact, there's everything right with it.

But there's a difference between commentary and investigative reporting.

GEOFF BENNETT: You spoke to 200 Democratic insiders over the course of your reporting.

More than 200.

How did you vet their motivations to make sure that they weren't engaging in sort of retrospective blame-shifting after the fact?

JAKE TAPPER: That's a great question.

Obviously, talking to as many people as possible so that you're not just relying on one person's point of view.

You're relying on more than 200 and you're really trying to get an assessment of what's real and what's not.

But one thing that almost all of them copped to was the fact that this was a disaster, it was hidden from the American people and that it can't happen again.

I think a lot of them felt they were unburdening themselves.

A lot of them felt they regretted not talking to reporters ahead of time.

But, by the same token, the central justification for the Biden candidacy was he's the only one that has ever beaten Trump and Trump poses an existential threat to the country.

If you convince yourself that those two statements are true, you can justify anything.

Once the threat of the election was over, a lot of them came and talked to us, and the result is the closest approximation to the truth of what happened during those four years.

GEOFF BENNETT: A spokesperson for the former president told me this.

He says: "There is nothing in this book that shows Joe Biden failed to do his job, as the authors have alleged, nor did they prove their allegation that there was a cover-up or conspiracy.

Nowhere do they show that our national security was threatened or where the president wasn't otherwise engaged in the important matters of the presidency.

In fact, Joe Biden was an effective president who led our country with empathy and skill."

To which you say what?

ALEX THOMPSON: We're not alleging anything, that this -- what is in this book is based on the interviews the people inside his administration, top Democrats throughout Congress, and members of their own Cabinet and White House.

They're the ones that told us that his age was affecting potentially his ability to do the job.

GEOFF BENNETT: What's your response to critics who say that this book punches at the wrong target at the wrong time, that focusing on Biden's decline distracts from Trump's threat to democracy, his authoritarian rhetoric, his attempt to dismantle institutions he doesn't like, so on and so forth?

JAKE TAPPER: All the stuff we cover every day.

ALEX THOMPSON: Well, and I would say, I mean, we write this in the book that, just because we are writing this about Joe Biden, it does not excuse anything Donald Trump is doing.

I'd also say if you're a Democrat wondering, how did Trump come back, in some ways, the central question that this book is answering is, how did Trump become president again?

And our belief is that Joe Biden deciding to run again was one of the critical, if not the critical part of his ability to return.

JAKE TAPPER: There are a lot of Democrats who think that the reason why the Democratic Party's approval ratings are so low and continue to be so low, even when Trump's numbers start going down, is because the American people feel lied to by the Democratic Party.

They feel misled because of the fact that the party was telling them, Joe Biden's fine, Joe Biden's fine, even though they, as you noted, in polling suggested, I don't know, I think he's too old to do the job.

And they kept saying, no, no, no, he's fine, he's fine, he's fine.

Then the debate happens and they are -- it's shown that they have been gaslighting the American people.

I think that there needs to be a reckoning in the Democratic Party to acknowledge this, not unlike the way that the Republican Party had a reckoning, and it took years, on the Iraq War and the weapons of mass destruction lie that was sold to the American people that was deadly.

GEOFF BENNETT: On the eve of this book's release, the former president announced that he has an aggressive form of prostate cancer.

There are those who feel this is hitting a man when he's down, he's no longer in office.

Why push forward with this?

ALEX THOMPSON: Because we felt there was an important story to tell about -- that we could only tell after the election.

And so in some ways it's not just for the moment, but we felt it was important first draft of history.

JAKE TAPPER: Yes, look, obviously our hearts go out to the Bidens.

I don't think the book is mean.

I don't think that we're unsympathetic to all of the horrible things he has endured throughout his life.

We have a whole chapter in it about everything he's gone through and the remarkable ability that Joe Biden has shown to get up and keep going on, despite so many horrible twists of fate that have been thrown at him.

But, that said, the American people deserve transparency when it comes to the health care of their leaders, whether Donald Trump or Joe Biden or whoever comes after Trump.

And there's never going to be a better time to tell that story than right now.

GEOFF BENNETT: Jake Tapper, Alex Thompson, the book is "Original Sin."

Thank you so much for being here.

ALEX THOMPSON: Thank you.

JAKE TAPPER: Thank you.