As President Trump and his administration attempt to undermine the claims made in an explosive new book about the inner workings of the White House during his first year in office, at least one former aide has confirmed his comments are authentic.

Sam Nunberg, an adviser on Trump's presidential campaign who was fired in August 2015 after racially charged Facebook posts he wrote surfaced, told ABC News that he "certainly, probably" called Trump an idiot in a conversation with former chief strategist Steve Bannon that was detailed by Michael Wolff in his new book, Fire and Fury: Inside the Trump White House.

"I certainly, probably said that but he's by no means an idiot, at all," Nunberg explained Friday morning on ABC News' The Briefing Room, saying he wasn't familiar with the exact quote.

In the book, which is selling out fast in the Philadelphia region, Wolff wrote that Nunberg told Bannon, "If you can get this idiot elected twice… you would achieve something like immortality in politics."

Nunberg said on ABC News: "I did sit down with Michael. I'm not out here to criticize Michael, but I think Michael used flourish, I'll put it, on the events I described."

In another of the book's pointed moments, Wolff wrote that, during the campaign, Nunberg attempted to teach then-candidate Trump about the Constitution, something the former aide said the businessman and media personality didn't understand and didn't want to learn about.

"I got as far as the Fourth Amendment, before his finger is pulling down on his lip and his eyes are rolling back in his head," Nunberg is quoted as saying in Wolff's book.

On ABC News, Nunberg didn't dispute the quote, and instead tried to put it in a broader context.

"The president had a granular understanding of the Constitution. It was good enough," Nunberg said, adding that he was attempting to protect the candidate from "gotcha" questions about the Constitution. "Whether he knows the Constitution or not, I cannot be happier by the judges he's appointed."

Wolff has been accused in the past of embellishing his reporting, and some have voiced skepticism about the juicy book's claims. The now-defunct media watchdog Brill's Content wrote about several factual errors in Wolff's 1998 book Burn Rate, including claims by 13 characters portrayed in the book who said Wolff "invented or changed quotes."

The writer responded to critics on NBC's Today show, telling co-host Savannah Guthrie that he reported the book the same way others have covered the Trump administration.

"I work like every journalist works, so I have recordings, I have notes," Wolff said. "I am certainly and absolutely, in every way, comfortable with everything I've reported in this book."

Former White House Deputy Chief of Staff Katie Walsh told Yahoo! News' Jon Ward that she planned to dispute some quotes attributed about her, but has yet to release a statement or comment further on them. Walsh is quoted by Wolff as saying working with Trump was "like trying to figure out what a child wants." Axios' Mike Allen reported that Wolff has dozens hours worth of tapes to back up his quotes, including some featuring Walsh.

"From what I've read so far, my view is that we should interpret the book as a compendium of gossip Wolff heard," wrote Vox's Andrew Prokop. "How much of that stuff is actually true is a different question — one that's much tougher to answer."

The most-newsworthy portion of the book thus far — Bannon's comments about a campaign meeting with a Russian lawyer in Trump Tower — appears to be accurate. Bannon hasn't disputed the quotes, and his website, Breitbart, ran them word-for-word in its story about excerpts obtained by the Guardian.

Thursday night, Trump wrote on Twitter that he "authorized Zero access to White House" for the author and "never spoke to him for book," but Wolff told Guthrie he "absolutely" spoke with Trump on the record several times.

"Whether he realized it was an interview or not, I don't know, but it certainly was not off the record," Wolff said. "I spoke to him after the inauguration, yes … I spent about three hours with the president over the course of the campaign and in the White House, so my window into Donald Trump is pretty significant."

Wolff also said that "100 percent of the people around" Trump, including his most senior advisers and even family members, "questions his intelligence and fitness for office."