"I don't like him", the elder Bush told author Mark K. Updegrove in May 2016. "And I'm not too excited about him being a leader", he remarked.
Bush Sr., who voted for Hillary Clinton in the 2016 election, thought Trump seemed to have "a certain ego" but initially had hopes that he could unite the country if he possessed a sense of "humility".
During a heated Republican debate past year in which Trump exchanged blows with another member of the Bush clan, the President blamed George W. Bush not not only for the quagmire in Iraq but also the 9/11 terror attacks. Son George W. Bush also gets in on the criticism, believing that Trump "fans public anger and came to office without any understanding of the job", according to the report.
Both former presidents have candidly voiced concerns about Trump's leadership ability.
And both worry that Trump has blown up a Republican Party that they spent two lifetimes building, a party that was once committed to removing boundaries to trade and immigration, promoting democracy and civil society and asserting a robust USA leadership role in the world.
Neither the 41st or 43rd presidents voted for Mr Trump in the election. Bush confided to the author, "I'm anxious that I will be the last Republican president".
According to the book, neither Bush voted for Trump, with the elder Bush saying he voted for Hillary Clinton. But under Updegrove's analysis of their values and those of past Republican presidents compared to Trump's exhibition of Republicanism, the title still stands.
"The Last Republicans", published by HarperCollins, will be out November 14.
In the book, Mr Updegrove writes that the younger Bush did not think Mr Trump would win when he first entered the presidential race.
"If you look at the Bush family, it makes flawless sense". The Bushes felt stung by Trump's ground-burning attacks that helped destroy the campaign of Jeb Bush, the son and brother of the presidents.
Just last month, George W. Bush delivered a speech that seemed to call out so-called Trumpism - and the bigotry, conspiracy theories and "casual cruelty" associated with it. George W. Bush told Updegrove he left the top of the ballot blank.