This is a tragedy. Just when a route, albeit tortuous, to Britain’s exit from the European Union finally lies open, a fatal miscalculation by Brexit’s greatest champion has put that departure in serious jeopardy. Nigel Farage’s ultimatum to the Conservatives which, if framed in different terms, might have concentrated Brexiteer minds and successfully forged a Leave alliance now threatens to destroy Brexit.

It would be a great irony of history if the man who forced an EU referendum on the establishment and played a significant role in winning it (despite Tory denials) were to become the individual who unintentionally forced Britain back into the maw of the European Union, with no chance of any future referendum on withdrawal.

At the launch of the Brexit Party’s election campaign Nigel Farage made the worst call of his life. That criticism does not derive from the all too familiar media orthodoxy that Farage is someone to be denounced, mocked and despised. On the contrary, by any objective assessment (though there is precious little of that around), Nigel Farage is one of the most principled and successful pro-democracy activists in Europe.

He transformed UKIP from a bunch of nineteenth-hole grognards into a successful political force. He compelled the establishment to hold a referendum on EU membership and, after that plebiscite was won, demonstrated his disinterestedness by retiring. If he has an ego, so does every other politician, few of them with as much justification as Farage.

But what makes Farage stand out against a crowded backdrop of hypocritical, unprincipled, lying politicians is his principled stance. People often ask: does Boris Johnson really believe in Brexit? Most conclude that he took his pro-Leave stance at the referendum in the expectation of losing but endearing himself to the Tory grassroots. People also ask: does Jeremy Corbyn believe in Remain? His lifetime record suggests he does not. The leaders of the two main parties are almost certainly acting against their private beliefs.

But nobody, not even among his countless enemies, has ever asked if Nigel Farage believes in Brexit. Everyone knows he believes passionately in leaving the European Union and has devoted much of his life to that cause. Unfortunately it is that very principled stance, his personal integrity, that has now brought Farage to the verge of becoming Brexit’s terminator rather than its deliverer; a man who, if his miscalculations keep Britain in the EU, would never be able to enter an English pub again.

He has made an electoral alliance with the Conservatives conditional on Boris Johnson abandoning the “deal” he has confected with Brussels. In principle, of course, Farage is right. The deal is 95 per cent Theresa May’s rancid treaty, with 5 per cent of the text improved. Though the public seems blandly unaware of its ghastliness   – even UK foreign policy would be controlled by Brussels, arguably to a greater degree than it has been under full EU membership – the Boris deal is not Brexit in any recognizable form. All things being equal, it should be rejected out of hand.

But all things are not equal – far from it. The sole merit of Boris Johnson’s treaty is that it is a gateway: it is the only means currently available of opening the prison door bolted shut by Brussels and its fifth column at Westminster, holding the United Kingdom in durance vile three and a half years after this supposed democracy voted to leave. Once legally outside the EU, no longer subject to anthem and flag, it would all be to play for in the subsequent negotiations.

If Boris Johnson can secure a majority, an exit on WTO terms could once again be, in that obnoxious phrase, “on the table” as a default position. Undoubtedly Michel Barnier would deal more cautiously with a genuinely pro-Brexit UK government with a parliamentary majority than he ever did with the Remain-inclined Theresa May and her forelock-tugging acolytes. Until Boris went to Europe in person to confront the EU establishment, British negotiators had resembled a Burghers of Calais tribute act.

Nigel Farage has made the fatal mistake of demanding the one concession Boris Johnson cannot make. With a general election campaign already under way, the Prime Minister cannot conceivably abandon the withdrawal agreement that is his only asset. It was to advance that agreement he forced an election. How could he now turn to the country and say: “I’m just going to scrap this deal and fight the election empty-handed, taking us back to square one on 24 June, 2016, in the Micawberesque hope that something will turn up.”

How persuasive an electoral programme would that be? It would make Labour’s Byzantine stance look clear-sighted and statesmanlike. Nigel Farage must be intelligent enough to see that he is asking for the impossible. If he wants a Leave alliance (and it is clear he really does), he must change the terms of his offer.

So, is it all Nigel’s fault, then? Are the Tories the much put-upon victims of Brexit Party intransigence? Far from it: for months, the Conservatives have played fast and loose with Brexit by haughtily advertising their contempt for the Brexit Party and their lofty disdain for even the mildest accommodation with Farage. That purblind attitude may partly be fuelled by Dominic Cummings’s dislike of Farage, but it also has the authentic resonance of the old Tory grandee insouciance towards their betrayed followers: “They have nowhere else to go.”

Unfortunately, by making an unreasonable demand, Nigel Farage has given those arrogant isolationists a pretext to plough a lonely furrow towards electoral defeat. For the notion that the Conservatives can win this election with Brexit Party candidates standing in every constituency – or even 15 per cent of constituencies – is a fatal delusion. The Tories are as much as 16 points ahead in some polls, chirp the optimists; very good – Theresa May was 22 points ahead at this stage in 2017 and that ended well, did it not?

This election is the most unpredictable in generations. It also has the most important issue depending on it since 1945: Britain’s sovereignty. Suppose the Brexit Party only has seven per cent support, that could be lethal to the Tories in many seats. The Conservatives are deceiving themselves in imagining they can win swathes of pro-Leave Labour seats in the north. They cannot. Culturally, they are toxic. Families in the north have voted Labour since 1918: they might abstain, they might well vote for the Brexit Party, but they will not vote for the immemorially demonized Tories.

If the anti-democrats who have shamelessly surfaced are to be prevented from reversing a referendum result – signalling the end of democracy in Britain – there must be an arrangement, however informal, between the Conservative and Brexit parties. Anything else is insane.

Nigel, please rethink your demands and formulate a reasonable proposal, or see your life’s work destroyed. Boris, please come down off your high horse and recognize that, on this occasion at least, Donald Trump is right: allied, even informally, the two pro-Brexit parties would be an “unstoppable force”.

Both Conservative and Brexit Party supporters must inundate their leaderships with urgent demands for cooperation to save Brexit and exclude Jeremy Corbyn from 10 Downing Street. The alternative will be a nightmare. Remorse on 13 December – Friday the Thirteenth – will come too late.

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