WASHINGTON - Former US president George HW Bush voted for Hillary Clinton in the 2016 election and called Donald Trump a "blowhard" who was driven by "a certain ego." His son George W Bush left his ballot blank and said of Trump: "This guy doesn't know what it means to be president."

The revelations, contained in historian Mark Updegrove's "The Last Republicans," available in bookstores 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 Bush told Updegrove in May 2016 before the elections in November of that year, according to excerpts of the book obtained by AFP. "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."

The younger Bush was skeptical about then-candidate Trump's chances in a race in which his younger brother Jeb was an early favorite. "Interesting, won't last," was his initial reaction when Trump entered the race.

Once the bombastic billionaire real estate magnate secured the Republican nomination, Bush expressed surprise and worried about Trump's lack of humility, and thus his inability to recognize his own limitations and surround himself with more knowledgeable people. "As you know from looking at my family," Bush said, humility "is a certain heritage, that's what they expect, and we're not seeing that" in Trump.