HOUSTON (CNN) -- Former President George H.W. Bush has a blunt assessment of Donald Trump: "He's a blowhard."
And his son, former President George W. Bush, has harsh words for his Republican successor as well: "This guy doesn't know what it means to be president."
Those stinging comments mark the first time the former presidents are speaking out about Trump in such stark terms, as part of a new book about the father and son by historian Mark Updegrove, titled "The Last Republicans."
Both men went on the record to give Updegrove their candid assessment of Trump, as well as rare insight into their thoughts on the 2016 presidential race as the drama unfolded.
The most critical comments about Trump came from the elder Bush, now 93, who told Updegrove in May 2016, "I don't like him. I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being a leader."
The author went on to ask George H.W. Bush what he thought Trump was seeking in running for president, and the former Navy war hero responded by saying that Trump had "a certain ego." Though George H.W. Bush told the author he thought Trump could unite the country, he said it would require "humility," which the former President told Updegrove was a challenge for Trump.
"If you look at the Bush family, it makes perfect sense. Donald Trump is everything that the Bush family is not," Updegrove told CNN's Jamie Gangel. "George Bush grew up thinking about the greater good. Donald Trump is manifestly narcissistic. It's part of his brand. And that brand is the antithesis of the Bush brand."
George W. Bush echoed his father's sentiments when he told Updegrove, "As you know from looking at my family, (humility) is a certain heritage. That's what they expect, and we're not seeing that (in Trump)."
Updegrove told CNN: "When Donald Trump said, 'I am my own adviser,' Bush thought -- and this is a quote -- 'Wow, this guy doesn't know what it means to be president.'"
In the book, Updegrove writes that the younger Bush did not think Trump would win when he first entered the presidential race. "Interesting, won't last" was the reaction that George W. Bush had in those days, Updegrove writes. And he added that the younger Bush was surprised when Trump emerged as the Republican Party nominee. "When you're not out there and you're not with the people, you don't get a good sense of (the mood),'" George W. Bush told Updegrove.
CNN has reached out to both former presidents' offices and they both confirmed that they had spoken with Updegrove on the record, but as a policy do not comment on books.
White House hits back
In response to the Bushes' comments, a White House official told CNN: "If one presidential candidate can disassemble a political party, it speaks volumes about how strong a legacy its past two presidents really had. And that begins with the Iraq war, one of the greatest foreign policy mistakes in American history."
"President Trump remains focused on keeping his promises to the American people by bringing back jobs, promoting an 'America First' foreign policy and standing up for the forgotten men and women of our great county," the official said.
On Hillary Clinton
In the end, neither Bush voted for their party's nominee. George H.W. Bush confirmed that he voted for Democratic nominee Hillary Clinton. George W. Bush told Updegrove he left the top of the ballot blank.
"I voted 'None of the Above' for president, and Republican down ballot in 2016," he said.
He went on to say that concern with the former secretary of state's judgment kept him from following his father's lead. He told the author in the weeks leading up to Election Day, "The question for the country to decide -- on both candidates, by the way -- is to what extent should we be insisting upon integrity and solid character."
George W. Bush described his relationship with Clinton, according to Updegrove, saying: "'In my presence, she was polite ... thoughtful," he said, but alluding to her using a private email server as secretary of state, he added, "obviously tangled up in bad judgment. This email thing, putting confidential information out there in a world where all kinds of people can figure out how to get your emails was not good judgment."
In the end, the younger Bush concluded it was "a strange election year," and told Updegrove both candidates were "among the few in the country" who made each other "viable."
The book includes other blunt comments by George W. Bush, including his reaction to stories about his rebellious youth and the perception that his vice president, Dick Cheney, exercised too much power.
In the first chapter, George W. Bush calls the notion that he was the "prodigal son," a reference to a wayward son in the Bible, "bullshit." And in a 2012 interview with Updegrove, Bush told the historian he "chased a lot of pussy and drank a lot of whiskey" as a young man, but added, "I was never the prodigal son because I never left my family."
The author told CNN he concluded that though Bush was rowdy in his younger years and his antics rose "to the level of sophomoric hijinks that might be the basis of fraternity lore," his rebelliousness has been greatly exaggerated.
This isn't the only notion Bush tries to dispel. When asked about the power and influence of former vice president Cheney and former Secretary of Defense Donald Rumsfeld in his administration, George W. Bush replied that Cheney and Rumsfeld "didn't make one fucking decision."
Discussing that quote, Updegrove told CNN, "I understand his frustration, because at the time there was the perception that Dick Cheney was the acting president. That Cheney was sort of a Machiavellian puppet master. But, in fact, George W. Bush had had a lifetime of making bold decisions. He has this natural confidence in himself as a leader. And if you talk to those around him, they have confidence in his leadership. And so, this notion that Cheney was making the decisions is ludicrous."
Father and son
The relationship between father and son is what drove Updegrove, who is a historian and former director of the Lyndon B. Johnson Presidential Library, to write "The Last Republicans."
"There are so many misconceptions about this relationship," the author told CNN. "You have to get the story right, and I felt in order to do that, you had to talk to the principles themselves." Updegrove spoke with both former presidents several times in the past few years in order to delve into their personal connection, which he repeatedly described as "very close."
Even so, the author said that George H.W. Bush was careful not to interfere when his son became President. "Because he understands what it is to be president, and he doesn't want to be a burden to his son," Updegrove told CNN. "So what he is, instead, is a comforter in chief. He's there for his son to be a loving presence in his life. That's what he considers his most important role."
The bottom line: Updegrove calls the relationship between the first father-son presidential duo in more than 150 years "a complicated love story, but a love story nonetheless."
As for "The Last Republicans," Updegrove said the idea came from a comment George W. Bush made after last year's election.
"George W. Bush himself said in 2016 privately, and then to me, that 'you know, I fear that I will be the last Republican president.' And it wasn't just about Hillary Clinton becoming president, as the Republican Party was having a difficult time finding itself. It was because Donald Trump represented everything that the Bushes abhorred," the author told CNN.
When asked his thoughts on George W. Bush's fear, Updegrove told CNN, "There's no question that I think there's a battle for the soul of the Republican Party. And the Republicans have to figure out who they are, and what they stand for."
"The Last Republicans" goes on sale on November 14.
By Javier De Diego and Jamie Gangel, CNN
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