WASHINGTON • Former United States president George H.W. Bush voted for Mrs Hillary Clinton in last year's election and called Mr Donald Trump a "blowhard" who was driven by "a certain ego".
His son George W. Bush left his ballot blank and said of Mr Trump: "This guy doesn't know what it means to be president."
The revelations, contained in historian Mark Updegrove's The Last Republicans, available later this month, represent the strongest criticism to date from the Bush clan on why their Republican successor is uniquely unfit to hold office.
"I don't like him," the elder Mr Bush said in May last year, before the elections in November, according to excerpts of the book. "I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being (our) leader."
While travelling to Tokyo on Air Force One, Mr Trump walked back to the press cabin and responded to the Bushes' criticism. "I don't need headlines. I don't want to make their move successful," he said.
The younger Mr Bush was sceptical about Mr Trump's chances in a race in which his younger brother Jeb was an early favourite. "Interesting, won't last," was his reaction when Mr Trump entered the race.
Once the bombastic billionaire real estate magnate secured the Republican nomination, Mr Bush expressed surprise and worried about Mr Trump's lack of humility, and thus his inability to recognise his own limitations and surround himself with more knowledgeable people. "As you know from looking at my family", Mr Bush said, humility "is a certain heritage, that's what they expect, and we're not seeing that" in Mr Trump.
I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being (our) leader.
FORMER US PRESIDENT GEORGE H.W. BUSH, on Mr Donald Trump in May 2016.
DOESN'T UNDERSTAND THE JOB
Wow, this guy really doesn't understand the job of president.
FORMER US PRESIDENT GEORGE W. BUSH, after Mr Trump said "I'm my own adviser".
After Mr Trump said "I'm my own adviser", Mr Bush quipped: "Wow, this guy really doesn't understand the job of president."
The comments are sure to escalate a long-running feud and come after Mr Bush gave a speech last month that condemned bigotry, bullying and lies in US politics in what was widely seen as a broadside against Mr Trump, even though he was not mentioned by name.
The book's title was inspired by the younger Bush's concerns that he had been "the last Republican president" - not just because Mrs Clinton was seen as favourite to win the election, but because Mr Trump represented a major break from traditional conservatism.
A White House official said: "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."