President Donald Trump's White House has responded to former Presidents George W. Bush and President George H.W. Bush, who gave scathing assessments of Trump in a new biography.

'If one presidential candidate can disassemble a political party, it speaks volumes about how strong a legacy its past two presidents really had,' a White House official told CNN on Saturday.

'And that begins with the Iraq war, one of the greatest foreign policy mistakes in American history.'

The rebuke came in response to interviews with the father and son ex-presidents in historian Mark Updegrove's new book, 'The Last Republicans'.

'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 elder Bush said of Trump in a May 2016 interview included in the book.

It also emerged that George H.W. Bush admitted voting for Hillary Clinton in the 2016 election, while his son left the top of the ballot empty and voted for Republicans down-ballot.

In a biography on the father-son presidential duo, the elder Bush calls Trump a 'blow hard'

The Last Republicans is out November 14, with on the record interviews with both Bushs, revealing how they feel about President Trump and Hillary Clinton

George W. Bush echoed his father's sentiments when he told the author Mark 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 explains he came to the tome's title after the junior Bush said he feared he would be the last Republican president- and not just because of potential Hillary Clinton win at the time- but 'because Donald Trump represented everything that the Bushes abhorred.'

This is the first time either of the men have given their frank assessment of Trump. According to CNN, the book will also reveal their rare insight into the 2016 presidential race- as it unfolded.

'If you look at the Bush family, it makes perfect sense. Donald Trump is everything that the Bush family is not,' Updegrove told CNN.

'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.'

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 younger Bush's reaction in the early days of the race, Updegrove wrote.

When it came time to vote in the 2016 race, the elder Bush voted for Clinton, while George W. Bush didn't trust her judgement enough, and checked the 'None of the Above' box

He was also 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.

When it came down to election day, neither voted for their party's nominee.

The younger voted 'None of the Above' for the presidential ticket, and voted Republican down the line afterwards. Meanwhile the elder Bush voted for Hillary Clinton.

In a revealing look at how former presidents decide on choosing a candidate, W Bush says when it came down to it, he simply didn't trust Clinton.

'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.'

Leading up to Election Day he told Updegrove '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.'

The younger also touches on how his youth has been considered rebellious, and feels it is an unfair assessment. In a 2012 interview with Updegrove, Bush told the historian he 'chased a lot of p**** 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 Last Republicans' goes on sale on November 14.