Protesting against Donald Trump’s state visit to the UK is, first, an act of elementary political hygiene: a refusal to endorse the British government’s eager servility to the United States, and a rejection of the politics of the president and his various global allies. Trump provokes a curious mixture of fascination and repulsion, however, and the reasons for protest go beyond a rejection of the current US government, to a sense that Trump presages a new and dangerous way of doing politics.

Trump isn’t the first US president to be greeted here this way: both Reagan and Bush Jr met significant protests when they came to Britain, but both nuclear proliferation and the conflagration in Iraq involved Britain as Washington’s reliable and loyal poodle. No such obvious culpability ties the British government to Trump, despite May’s eager transatlantic abasement within days of his inauguration. Fears linger that a post-Brexit Tory government will flog the NHS to American insurance vultures and flood Asda with chlorinated chicken. Some people point to nebulous links between Russia, Trump and Brexit, perhaps drawing comfort from the notion that all our social ills have their roots in the perfidious Kremlin.

There’s no shortage of specific reasons for objecting to Trump: the Muslim ban; the separating of children from their parents at the border; the claim that there were ‘very fine people’ among the fascists at Charlottesville; the wholesale attack on climate science and gutting of the EPA; the assault on women’s reproductive rights; the gradual crescendo of the drumming for war with Iran. Add to that incomplete list: the everyday lies, the Twitter monomania, the mind-warping crassness and vulgarity, the obvious infatuation with strongmen and distaste for democratic accountability, the grifting, the groping, the further enrichment of his grotesquely unqualified family – the sense, finally, that the feculent American id, fed on a diet of Fox News and Twitter conspiracy, now whines and glowers behind the Resolute desk.

Trump’s opponents often think such lists speak for themselves, as if the litany generates its own argument; but it can also produce numbness, paralysis or resignation. I took care to separate out the political priorities of the Trump government from the personal infractions of the man, but they often bleed into one another – as if to imply that Trump’s vulgarisation and demystification of the presidency were somehow equivalent to locking toddlers in cages. The presidency has always been shaped by and expressed through the personality of its holder, but Trump would like to demolish the distinction between man and office entirely. He cannot conceive of politics in terms other than those of personal loyalty. This personal element, combined with his inclination to authoritarianism, his racism and his openness to the alt-right, leads politicians and thinkers of the left to see parallels with 20th-century fascists, though most hesitate at making the bald analogy.

In the bastions of American liberalism, fear of Trump concentrates into disquiet over his supposed erosion of democratic norms. In the most distilled version of this view, the political specifics of the Trump presidency – the racism, the anti-environmentalism, the hostility to international institutions – disappear, as it becomes symptomatic of a more fundamental but curiously apolitical threat, sometimes called ‘polarisation’, sometimes ‘extremism’. It isn’t hard, mutatis mutandis, to see the same analysis being repurposed against a future president put in office by a left-wing movement.

You can hesitate at being conscripted into the liberal battle for the status quo ante, and bristle at facile historical analogy, but also recognise what is novel and dangerous about Trump. His style of politics isn’t fascist but – as Weber might have put it – patrimonial: hence his emphasis on loyalty, familial enrichment and rewards for his liegemen. He regards international institutions with a similar eye to recompense and vassalage, blithely expressing things his predecessors have tactfully left unsaid about the politics of Nato, or American trade priorities. Sometimes this is in open conflict with the bureaucracy of the American state; sometimes it dovetails with its already oligarchic cast. Then there is his media ubiquity, his strategy of direct communication with his base through rallies and Twitter, and his merry willingness to break taboos, attacking politicians and businessmen, or intimidating potential witnesses for Congressional inquiries.

And yet his electoral success depended not only on the most conventional of American institutions – the Electoral College – but on the longer term work of the Republican Party, from the ‘Southern Strategy’ onwards, of reshaping sections of the American electorate towards forms of atomised anti-solidarity, fundamentalist patriarchy, racial resentment and vituperative hatred of the fragmentary remains of the New Deal and the Great Society. The departments of government that have not been given over to Trump’s loyalist cranks are headed by standard Republican functionaries; most of the state’s bureaucracy continues unaltered beneath the patrimonial web, its military arm more autonomous than ever under a president impressed by its tinsel and firepower.

What makes Trump’s presidency so mercurial is his alternation between different, sometimes contradictory modes of ‘doing’ politics, held together by the force of his personality and the use of media spectacle. Similar formulae apply to the other leaders often bracketed with him: Erdogan, Orbán, Salvini, Modi.

If it is necessary for Trump’s opponents to defend such democratic virtues as deliberation, the separation of powers, and free and fair elections, it is also necessary to recognise their imperfections in practice and to call for their reform. Trump’s insistence on recognising, through his own warped sense of American victimhood, the political dimension of such international institutions as Nato and the WTO, the political power of the media and business, and the politicised nature of such institutions as the American judiciary, ought to be a gift to the left. In saying out loud what politicians tend to say quietly, Trump offers the opportunity for a left-wing opponent to broach what is normally taboo in political life, and challenge the claims of ‘apolitical’ technocracy. Anti-Trumpism can too easily oscillate between a recognition of the systemic problem and a hankering to re-establish the silence of the taboo.

For the protesters in London today, standing up to Trump may mean recognising that his blend of populism, nihilism and reaction is also present in the Tory Party (Boris Johnson, the favourite to replace Theresa May as leader, is its avatar). It may mean recognising the similarities between Trump’s campaign sloganeering and our own current exercise in post-imperial delusion. Perhaps the crowds in Trafalgar Square today will also help to erode a pernicious norm: that however base or corrupt, however politically retrograde a US president, Britain will always roll out the red carpet for him, calculating that trade advantage or membership of the nuclear fraternity trumps basic political dignity and decency every time. To those who regard this as an immutable axiom of British politics, today’s protests say: no more.