In politics, as in life, the things we value are more at risk from the rot we allow to set in than through any freak event.

In the days since the storming of the US Congress a number of UK pundits have rushed to stress that UK prime minister Boris Johnson is no President Donald Trump. But this happy and correct conclusion also highlights the fact that such reassurance is needed.

It is not that similar scenes are suddenly likely in the UK — one benefit of a parliamentary system is that it is easier to remove a rogue leader. It’s more that Britain has let some of the same rot set in. And while Mr Johnson’s attraction to unconventional means, albeit in pursuit of often mainstream goals, deserves much of the blame, the malaise runs deeper than one leader or party.

Consider some warning signs: deviation from key principles of democracy; opportunistic assaults on institutions; threats to breach an international treaty; the undermining of accepted truths; the incitement of polarisation; and a toxic blend of zealotry and complicity. All are stretching the elastic of British democracy.

This is not about the policy of Brexit. However mistaken that cause, it was a legitimate belief pursued through democratic means. For all of its distortions, the campaign was built on a central truth about the loss of sovereignty.

But the referendum’s aftermath saw both sides deviating from agreed norms. The actions of Brexiters were worse but did not take place in a vacuum. Pro-Europeans withdrew losers’ consent, seeking to overturn the outcome with a second referendum. Meanwhile, after legal setbacks, Leavers turned on the judges, their media supporters denouncing them as “enemies of the people”. Mr Johnson unlawfully suspended parliament and his attorney-general labelled it illegitimate as a prelude to a people vs politicians election. This dalliance with demagoguery powered an outlook in which ends justified means. What was once unthinkable became imaginable.

Mr Johnson’s team did not plan to upend the institutions of democracy. But the desire to win made them careless of the consequences. While some Conservatives approved, many mainstream MPs were horrified. And yet (like Senate Republicans) they saw that resistance offered demotion, deselection and defeat — so played along.

But complicity goes beyond Brexit. It is particularly a function of an electoral system that punishes smaller or breakaway parties. The easiest path to power for hardliners is to capture one of the two great parties, leaving moderates forced to leave or comply.

After the hard-left capture of their party, Labour moderates had to choose between the political suicide of splitting or staying to fight. But even for the resisters, the price of remaining was participating in the campaign to make Jeremy Corbyn prime minister. His defeat and replacement by the more mainstream Keir Starmer means the gamble paid off. But what if he had won?

These are extreme scenarios, but the culture of complicity is deeply rooted. Mainstream politicians of all sides have learnt to “play the game”. A close friend of George Osborne’s cites the former chancellor’s first rule of politics: “Identify the next Tory leader and stick to them like glue” — advice clearly followed by his protégé Matt Hancock, the health secretary.

What follows is the knowing perpetuation of falsehoods, though this is hardly unique to the UK. Clearly, the line between political hyperbole and untruths is blurred — but there ought to be a line. Contrary to Labour’s routine protestations, in no modern election have voters had “24 hours to save the NHS”. Equally, no honest Tory should be parroting the prime minister’s claim that his Brexit deal contains no non-tariff barriers to trade and no regulatory border in the Irish Sea.

Another parallel is the sanctioned assault on the media, the BBC especially. There is much wrong with the national broadcaster, but it still aims for honest coverage. Thus it finds itself in the crosshairs of Brexiters and Remainers, Scottish nationalists and Corbynites, always for the same crime of insufficient commitment to their truth.

The looming arrival of partisan news channels, notably a Rupert Murdoch station modelled on Fox News and with a commercial stake in polarisation, is a further step towards undermining shared truths and entrenching the intolerance of opponents — which is then used to justify unconventional actions. Mr Trump’s excesses were made possible by media forces happy to play up betrayal myths against Democrats and indulge his lies about a stolen election.

The warning for the UK is that complicity with often casual breaches of convention can place you on a dangerous trajectory. Newly partisan broadcasters are a combustible addition once you have signalled it is OK to dispute electoral outcomes, suspend parliament and denounce judges.

It is not that British democracy is suddenly in peril but that stability is being taken for granted. We are further down the US path of polarisation than we might wish — which is why a commitment to the rule book is so vital.

The lesson is that the UK needs to step back from the “whatever it takes” carelessness with the constitution and towards the shared restatement of the core principles and institutions of its democracy. The US has offered a vision of a future for countries that lose sight of those values.

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