More than 18 months into his presidency, Donald Trump’s modus operandi – and the danger it represents – is clear. His working method is that of the mafia boss and gangland chieftain, daily wielding his power to settle scores, teach lessons and crush dissent. Anyone who’s seen The Sopranos will know the routine: the casual intimidation, the obsession with loyalty, the brutal ostracism meted out to those who dare defy the man at the top.

'McCarthy-era tactics': ex-spy chiefs round on Trump in Brennan row Read more

This week’s demonstration came at the expense of John Brennan, the former head of the CIA who spent a quarter-century in intelligence and counter-terrorism. He’s no dove: his record includes rendition, drone strikes and illegal spying on the US Senate. But he has become one of the most trenchant critics of Trump – he accuses him of treason – and the president exacted his punishment, stripping Brennan of the security clearance that had given him access, even in retirement, to some of his nation’s secrets.

You can speculate as to why Trump did it and why he did it now. The answer to the first question was provided by the president himself. He contradicted the official line supplied by his own press secretary Sarah Huckabee Sanders – that Brennan was punished for “erratic conduct and behaviour” – to tell the Wall Street Journal that what really bothered him was Brennan’s early role in the “sham” Russia inquiry that is driving him to increasing fury. As for why now, this has been a wretched week for Trump, as former aide and Apprentice star Omarosa Manigault Newman published a book alleging, among other things, that tapes exist of Trump using the N-word. Casting out Brennan served as a useful distraction.

On the face of it, this might not look like such a hardship for the former CIA boss. An estimated 4 million Americans have security clearances of one form or another. Losing his might diminish Brennan’s value as a consultant in the private sector; on the other hand, it boosts his stock as a media pundit. But that’s hardly the point. Determining access to classified information has, until now, been a non-partisan, administrative matter. Trump has used it a political tool, which is why on Thursday no fewer than six former CIA chiefs signed a statement denouncing the move. In the words of ex-CIA officer David Priess, this “is something that happens more in a banana republic than the United States of America”.

To be fair, the US is not wholly a stranger to such behaviour. When Sanders named eight other former public servants now similarly threatened with losing their security clearance – all of them connected with the Russia probe, funnily enough – she evoked memories of Richard Nixon’s notorious “enemies list”, a place on which fast became a badge of honour. The parallel is not fatuous: Nixon’s great offence was abuse of power, and this is becoming Trump’s hallmark.

Consider his ongoing war on what he brands the fake news media. That goes beyond “enemy of the people” rhetoric, or denying access to those whose coverage he dislikes. Instead Trump seeks to use federal powers to whip a dissenting press into line. Note his repeated threats to increase the postal rates paid by Amazon: you don’t have to be a fan of that company to see that what Trump really seeks is to punish Amazon’s boss Jeff Bezos for the hostile coverage dished out by the Washington Post, which Bezos also owns. Trump has similarly mused about denying a broadcast licence to NBC, and tried in vain to thwart a merger of AT&T and Time Warner, infuriated by the latter’s ownership of CNN, which he loathes. In each case, Trump took what had previously been regarded as a neutral function of government and abused it to punish those he deems political enemies.

Trump won't recognise there are spheres of administration that are meant to operate free of executive interference

Trump flatly refuses to recognise that there are spheres of administration that are meant to operate free of executive interference. He believes instead that everyone who works for the US government works for him and his family. Recall his reported demand that former FBI director James Comey declare his loyalty not to the constitution but to Trump. Or his shock that the attorney general did not act as his personal lawyer, making the Russia probe go away, but instead recused himself from it. How telling that even his tribute to Aretha Franklin included the tactless and apparently groundless claim that “she worked for me”.

The picture of Trump as president is now crystal clear. His instincts and methods are those of the autocrat. He respects no separation of powers, no zones of authority from which the constitution very deliberately excludes him and his office. He may be called Donald, but he wants to rule like a don.

The danger of all this to Americans is obvious. The US system of government, cherished and nurtured over two centuries, is being eroded by a president who tramples over every convention and custom that ensures its survival – and, crucially, by his Republican enablers in Congress who could stop him but won’t. (In a chorus, they supported his act of revenge against Brennan.)

Americans need to guard against an authoritarian impulse whose existence in their body politic is now demonstrably real. A survey this month found that 43% of Republicans were willing to give Trump the power to close down media organisations, while a separate poll a year ago found 52% would support “postponing” the 2020 election if Trump proposed it. Among all Americans, support for rule by the army – as opposed to elected politicians – is unusually high, with nearly one in five in favour.

But there is a threat here to the rest of us too. For Trump is forging a template for the 21st-century autocrat. Of course, there are already plenty of models to work from – Vladimir Putin, Recep Tayyip Erdoğan, Xi Jinping, Rodrigo Duterte – all of whom have received lavish, fawning praise from Trump. But an American president stands on a uniquely influential platform, observed the world over. Where once the US presidency offered an example of executive power restrained by the “co-equal” judicial and legislative branches, today Trump stands as an inspiration to every would-be strongman and abuser of authority, ready to bellow the line from his 2016 convention speech that could serve as a governing credo for tyrants everywhere: “I alone can fix this”.

Every time he steps over a once taboo boundary, thereby erasing it, Trump acts to normalise autocracy in the US and beyond. Rulers in Budapest and Warsaw, as well as Ankara and Moscow, see what Trump gets away with and they take note and take heart. He is a role model for the international strongman set. Which is why all those who care about global democracy should be praying for Trump’s Republicans to take a thorough beating in November’s midterm elections. As any mafia boss will tell you, the surest way to defeat a would-be strongman is to make him look weak.

• Jonathan Freedland is a Guardian columnist