Tea Party madness has brought the US to the brink of economic mayhem, risking taking much of the world with it. In the face of obdurate unreason, the president of hyper-reasonableness was forced to surrender. The economic credibility of the country that holds the global reserve currency has wobbled. The political credibility of the world's beacon of democracy has failed in the face of an insurgency of unreason. Facts, evidence, probability, possibility – none of that matters to a movement founded on ferocious fantasy.

The founding fathers built a constitution of checks and balances believing reasonable men would agree; how could they foresee Sarah Palin, Michele Bachmann or Glenn Beck? To the British eye, America was always dangerously prone to waves of populism and McCarthyite panics. The country has reached a deadlock that may set it on a faster road to decline as absolute intransigence creates a constitution that no longer functions. Why bother with the great show of presidential elections when presidents are denied the power to match their pomp? The politics of miasma, where words matter more than facts and actions, lets the Tea Party demand the impossible – debt reduction with tax cuts, spending cuts without touching the gargantuan defence budget. Obama believed against all the evidence that his opponents would see reason. That's not who they are.

I worked in Washington during Watergate and the fall of Richard Nixon; even in that national trauma there was not this unbridgeable detestation between the red and the blue. What happened? The rise of the Tea Party owes a great deal to Rupert Murdoch's Fox TV, the foghorn of extremism that changed the nature of political discourse. Trouncing the competition, its propagandising for Tea Party views misinforms the electorate on just about everything: it is rivetingly frightening viewing. It makes our own politics look civil, our commentating measured, our right wing moderate. But there is little doubt that had News International not fallen so spectacularly from grace, the Murdochs would have intimidated British politicians into changing our laws to allow unbridled political bias in broadcasting. Fox-style television would have battered its way into our living rooms, bringing us Tea Party politics too.

Whatever you think of the Tory party, it is not shot through with US craziness, not on stem cell research and gay marriage, or even really on abortion – though they will toughen its conditions. Steve Hilton's cunning plan to abolish all consumer, employment and maternity rights got a dusty answer, while his green passions are at least tolerated. Most Tories are driven by Thatcherism, with its shrink-the-state, on-your-bike thirst for deregulation. But although Oliver Letwin's parents were Ayn Rand disciples, the American right's call of the wild is no closer to Tory core sentiment than is Labour's ritualistic singing of the Red Flag once a year. Britain is more rightwing than mainstream Europe, our media more strident, but we haven't crossed the Atlantic – yet.

But American intellectual fashions waft our way: a taste of the Tea Party arrives on these shores in the peculiar paranoia of the climate-change deniers. You may dismiss some as fruitcakes or oil company lobbyists, but when Andrew Turnbull, former head of the civil service, reveals that he is of their number, it should alarm us.

Professor Steve Jones's report on BBC science coverage raised the difficult question of impartiality: should the BBC stand impartially between sense and nonsense, between flat- and round-earthers? On the MMR/autism dispute and GM crops the BBC gave a "false balance" between minority views and the consensus of most scientists. Jones suggests that the great weight of international scientific opinion agreeing that warming is caused by human agency means the BBC need no longer quote balancing deniers when only "the pretence of debate remains". Instead, move on to the real debates on how best to mitigate it. Mail and Telegraph commentators called this the "quasi-Stalinist thought police". For some reason they consider "the warmists" a leftwing conspiracy, though why is never clear. Lord Turnbull, writing in the Sunday Times, challenged Jones using every weary denier's argument: didn't Galileo and Darwin oppose the science of their day? I won't rehearse the paranoia of the deniers who think the world is against them: yes, it is.

On matters of fact, those of us who are not scientists can only listen to what scientists say and trust such an overwhelming global consensus. As cabinet secretary, Turnbull would have had to appraise evidence on myriad subjects of which he could know little: relying on best expertise is the only rational approach. So in what part of his psyche resided the Tea Party idea that scientific facts don't matter? Our senior civil service prides itself on drawing on the finest Oxbridge minds because they should be trained in evidence-based thinking. Turnbull was in charge of the civil service at the start of the Iraq war: on his watch the evidence in the notorious dossier was used to dragoon public support.

Reason should rule, but none of us is as rational as we pretend, each inhabiting our imaginations more than we do the real world, with opinions driven by beliefs, passions, convictions, hopes, fears and a hundred contradictory thoughts and impulses. But to make sense of the world, there is an obligation to seek out evidence and trust to expertise. Where it conflicts, we fight our political corners.

But science is different. Chief scientist John Beddington said in a forthright speech this year that we should become "Grossly intolerant of pseudo-science, the cherry-picking of the facts and the failure to use scientific evidence and scientific method". Repudiating evidence is Tea Party thinking – and we would do well to challenge its every manifestation in this country, above all in the seats of power.