“Bullying and prejudice in our public life sets a national tone, provides permission for cruelty and bigotry, and compromises the moral education of children,” he said at another point. “The only way to pass along civic values is to first live up to them.”

The remarks were an exceptional breach of the protocol governing post-presidential behaviour – be seen and rarely heard – and were especially striking coming from Bush. He has gone to great lengths ignoring repeated provocations from Trump, who savaged the former president’s younger brother Jeb in the 2016 campaign and often assailed Bush’s administration.

“He has, as matter of principle, made it a point not to comment on ongoing matters of political interest. I actually heard him in person talk about this,” said Russell Riley, co-chairman of the Presidential Oral History Program at the University of Virginia’s Miller Centre. “I think he wouldn’t do this unless he felt obliged to do so.”

Some who know Bush suggest he reached a breaking point and felt it no longer tenable to ignore Trump’s daily trampling of political and presidential norms.

“I think Trump has stretched the bounds of decency to a point where Bush is highly offended on the part of the nation,” said Don Sipple, a political ad maker who helped elect Bush governor of Texas and has periodically been in touch with the former president. “Bush is a traditionalist, and I think he thinks the presidency is being besmirched.”

Bush delivered the stinging rebuke of his fellow Republican as part of a larger survey of the political horizon at home and abroad. Speaking at a programme hosted by the George W. Bush Institute, his policy think tank, he restated many of the principles that guided his presidency and were standard GOP orthodoxy until Trump upended the party.

Bush celebrated the virtues of free trade and robust engagement around the world, saying they have contributed to stability and prosperity for the better part of a century. He acknowledged, however, a backlash as some have fallen victim to the destabilisation and economic costs of globalisation.

“In recent decades, public confidence in our institutions has declined,” Bush said. “Our governing class has often been paralysed in the face of obvious and pressing needs …. Discontent deepened and sharpened partisan conflicts. Bigotry seems emboldened. Our politics seems more vulnerable to conspiracy theories and outright fabrication.”

Hours before the speech, Trump suggested via Twitter that the FBI, Russian officials and the Democratic Party colluded to create a dossier of potentially incriminating information about him during the 2016 race. It was another unfounded theory promulgated by the president.

At one point, Bush seemed to allude to the white supremacists who marched last summer in Charlottesville, Virginia, under the Nazi creed “blood and soil.” Trump was widely assailed for his equivocal response to the violence provoked by members of the Ku Klux Klan and their sympathisers.

“Our identity as a nation – unlike many other nations – is not determined by geography or ethnicity, by soil or blood. Being an American involves the embrace of high ideals and civic responsibility,” Bush said. “People of every race, religion, and ethnicity can be fully and equally American. It means that bigotry or white supremacy in any form is blasphemy against the American creed.”

Bush also weighed in on Russian interference in the 2016 election, something Trump has steadfastly refused to acknowledge.

“Foreign aggressions – including cyberattacks, disinformation and financial influence – should not be downplayed or tolerated,” the former president said. “This is a clear case where the strength of our democracy begins at home. We must secure our electoral infrastructure and protect our electoral system from subversion.”

The indictment was unlike any in the modern history of the presidency, when those who have held the office abided by a mutual if informal agreement not to publicly criticise their successors. By failing to mention Trump by name, Bush maintained at least a patina of that precedent.

Stuart K. Spencer, who has spent more than 50 years in Republican politics as chief strategist for President Ronald Reagan and informal counsel to others who held the office, said Bush “was probably very upset, and maybe even irate” over Trump’s actions.

Not, Spencer said, that his cutting remarks are likely to change the president’s pugnacious approach.

“Not for a second. Not for a millisecond,” Spencer said. “I think it’s pretty well established how he handles himself. He’s going to do what he wants to do.”