There was a time when conspiracy theories were confined to the lunatic fringes of British politics. But like any form of erosion, they are making their way towards the centre – the grounded sanity of normal democratic politics is at increasing risk of subsidence.

Take a few examples. Alex Salmond has regularly encouraged the SNP myth that the British government kept oil fields secret from the Scottish people to discourage independence – most recently in his valedictory speech at his party’s conference. Troublingly, at the height of the referendum debate YouGov found that 26% of Scots believed MI5 was trying to prevent a yes vote. The SNP’s former deputy leader, Jim Sillars, publicly supported the idea. Disgracefully, Salmond himself gave a nod-and-a-wink response. “I haven’t met them thus far … as far as I know,” he said. This was a supposedly serious leader indulging in the politics of the tin foil hat.

In the aftermath of the referendum itself, out-of-context clips that wrongly purported to show vote-fixing in the count spread like wildfire – encouraged by a Russian observer and Sillars once again.

The virus isn’t just spreading among the Scottish nationalists. Ed Miliband sounded like a bit-player from the X-Files when he recently warned of “powerful forces” who wanted to keep him out of power. Leaving aside the most powerful force who want to do so, the electorate, his audience made clear who they thought he meant as they proceeded to heckle and abuse journalists who dared to ask challenging questions.

Scratch the surface and it’s disturbing how common it is in Labour circles to believe that Britain’s newspaper editors gather in darkened rooms to plot the publication of Miliband’s many gaffes. Some, it seems, simply cannot accept that maybe their leader’s poor reputation is of his own making.

That’s only one element of the left’s journey down the rabbit hole. Hatred of America, capitalism and the west in general has driven some to ever more disgusting acts of contortion to accommodate its enemies. Chomskyite moral relativism has devolved further into blatantly denying the true nature of tyrants around the world – why bother examining the possibility that local cultures excuse brutality and oppression when you can simply claim it is all made up?

Pussy Riot – no CIA stooges or members of the Vast Right Wing Conspiracy – recently spoke out to warn the British left against swallowing the “Kremlin propaganda” pumped out by Russia Today. Understandably, they’re concerned to see supposed progressives backing the tyrant whose gay-bashing, journalist-oppressing regime threw them in jail.

Sadly, Putin apologism isn’t confined to the left. Not only did Nigel Farage admire the former KGB man as “an operator”, also his party is convinced that Moscow is the victim of an imperialist Brussels. Apparently unconcerned by Kremlin-backed rebels shooting down a plane carrying British citizens, it routinely alleges that it is the EU’s fault that Russian arms, tanks and men have been pouring into eastern Ukraine.

Nor is the coalition government entirely immune. It beggars belief that until recently we had a Home Office minister, Norman Baker, who had written a book peddling the conspiracy theory that the security services murdered David Kelly.

The waves of effluent continue to lap at our political life from every side – fans of Julian Assange who think the Swedish judiciary is an outpost of the Pentagon, Labour MPs who genuinely believe cackling ministers are handing over ownership of the NHS to donors, Russell Brand telling Evan Davis on Newsnight that he was “open-minded” about the idea 9/11 was perpetrated by the American government.

This is not merely ridiculous, it’s dangerous. Rather than deal with the real dangers of the world, some people demonise the relatively free, democratic west because it suits their dogma to do so. Thinking themselves sceptics, they adopt a position of total gullibility.

When such self-deceiving practices were confined to the fringe they were a source of amusement. But now they creep closer to the mainstream they are a cause for concern. If a democracy’s people refuse to accept the outcome of elections, or its politicians prefer comfortable excuses to hard truths, every liberty we enjoy is at risk.