here have been some spectacular U-turns from political observers in the past five years – Piers Morgan’s desperate and tragically belated efforts to distance himself from Donald Trump, for example – but no reverse-ferret has been quite so vehemently trumpeted as that of Peter Oborne. Back in 2016, in his Daily Mail column, Oborne was proclaiming a new dawn of Conservatism, with Labour in collapse and David Cameron a busted flush. A “glittering prospect of 12 uninterrupted years as prime minister” awaited the winner of any leadership campaign, he suggested, and Boris Johnson’s years as mayor gave him “huge credibility” for the role. When the Brexit referendum got under way Oborne confidently announced: “In my opinion, Michael Gove and Boris Johnson are the two most brilliant politicians of their generation… courageous men… [with] the personal charisma and intellectual gifts to ensure that the case for Britain to leave the EU is seriously heard,” which “anyone who is a patriotic Briton – and everyone who believes in democracy – should welcome”. How to square that unqualified endorsement with the blunt question he asks at the beginning of this short and entertainingly outraged book: “What led the British people to put a liar into Downing Street?” A large part of the answer to that question Oborne lays at the door of “mainstream newspaper reporters and editors” who “collectively turned a blind eye to the lies, misrepresentations and falsehoods promoted by Johnson and his ministers” in order for him to bluster his way to power. Certain political correspondents are identified as having given Johnson an easy ride – Laura Kuenssberg of the BBC and Robert Peston of ITV among them. You get a long way into this book to find any genuine soul-searching on Oborne’s part, however – the first hint of mea culpa, “I start with self-reproach”, comes on page 137 – or any real analysis of how the Mail in particular refused to examine the false promises put forward by Johnson’s Leave campaign, while daily stoking wild fears about immigration. “There is only one good reason to be a journalist,” Oborne writes, portentously, “to tell the truth.” The case he makes against Johnson and Trump (a man whose election Oborne celebrated in the Mail with a piece headlined: “At last! He may be a bigot, racist and misogynist but Donald Trump’s revolution could finally bring back family values”) is the familiar one, an examination of a brutal kind of politics in which “falsehoods remain contemptuously uncorrected”, civil servants are bullied and hounded out of office and the very idea of facts is discredited or manipulated. He lays out the case for the prosecution against Johnson in some detail – the flagrant disregard of rules in reports from his Eton masters; the sackings for making stuff up, first at the Times, then as a shadow minister; the catalogue of lies in the 2019 election campaign, about new hospitals and police numbers. “Johnson,” he writes, “is Trump’s genteel country cousin, able to sugar-coat his lies with the legacy of an expensive classical education.” Over the course of Sherborne-educated Oborne’s career – he was hired by Johnson as his political editor at the Spectator in 2001 – he’d had ample opportunity to judge the suitability for high office of his former boss. Part of him still seems oddly, tribally, reluctant, even in the course of his argument that Johnson’s ingrained mendacity currently threatens to “destroy the country”, to give up the faith that Johnson was also “the most brilliant political journalist of his generation with a talent that at times crossed over the line to genius”. In order to find a way past the contradiction this suggests (isn’t journalism supposed to be about truth-telling?) he excuses the infamously fabricated news stories that the prime minister once offered about Brussels bureaucracy with the bizarre suggestion that Johnson injected “gonzo journalism into mainstream British political reporting” by being “truthful about his responses” to facts, rather than about facts themselves. Up until about spring 2019, it seems, Oborne continued to be cheerfully taken in by this music hall act. It was then, in a piece widely commended for its candour – and written not long after the departure of arch-Brexiter Paul Dacre as Mail editor – that Oborne admitted he may have got it badly wrong over Brexit and Johnson, while launching an attack on political lobby journalists “peddling Downing Street’s lies and smears”. His scathing insider’s analysis of that culture here is required reading for anyone interested in that world of anonymous sources and private briefings. There is a bracing convert’s zeal to his attack on a polarised media in which, as Orwell wrote of the Spanish civil war: “Everyone believes in the atrocities of the enemy and disbelieves in those of his own side, without ever bothering to examine the evidence.” Indeed, this cause now seems so heartfelt for Oborne that even an uncharitable reader might be moved to think: better late than never. • The Assault on Truth: Boris Johnson, Donald Trump and the Emergence of a New Moral Barbarism by Peter Oborne is published by Simon & Schuster (£12.99). To order a copy go to guardianbookshop.com. Delivery charges may apply