Cry #NeverCorbyn. Cry #NeverBrexit and you soon realise Britain is now a #NeverNeverLand of self-cancelling double negatives. The only way, it seems, to stop one extremist in #NeverNeverLand is to vote for another. The only way to save #NeverNeverLand from a rightwing disaster is to vote for a leftwing disaster. If you believe in fairies, Peter Pan says clap your hands and Tinker Bell won’t die. When set against what the British are being asked to believe in the general election campaign, belief in fairies sounds modest.

In a plea that might have been made by the anti-fascists of the 1930s, the Jewish Chronicle asked all citizens to walk in the shoes of British Jewry and not vote for Jeremy Corbyn. It asked: “How can the racist views of a party leader – and the deep fear he inspires among an ethnic minority – not be among the most fundamental of issues?” So devoutly does the former Labour minister Ian Austin believe in #NeverCorbyn that he is telling “decent, traditional, patriotic Labour voters” to support Boris Johnson at this election.

The trouble is that #NeverCorbyn and #NeverBrexit are incompatible. In many constituencies, the only way to stop Brexit is to vote for the Labour candidate, assuming he or she is an honourable pro-European, which, looking at the thugs and fruit loops Labour is offering the citizenry, may be an assumption too far. The Liberal Democrats, Plaid Cymru and Greens have vowed to unite to defend our membership of the EU. But their offer to put forward unity candidates is redeemable in only 60 seats.

Elsewhere, the Lib Dems will fight to destroy the few decent members of the parliamentary Labour party left: Rosie Duffield in Canterbury, for example, and Neil Coyle in Southwark. Labour supporters, meanwhile, will punish the Lib Dems for austerity rather than stand aside in the interest of removing Jacob Rees-Mogg in Somerset or Dominic Raab in Surrey.

People cling to their tribe, however ugly and dangerous its leader is. This is Johnson and Corbyn’s best hope

The absurdity of Britain’s position was captured by the young political writer Adam Barnett. Millions will reluctantly say #NeverCorbyn and support Johnson, while accepting that he lies as easily as others breathe. “Vote for the liar because he will deliver on his promises” will be their slogan. Millions will reluctantly say #NeverBrexit and support Corbyn, even though they know he is an ally of racists who has supported Brexit since entering parliament. “Vote for the Brexiter antisemite to stop Brexit and deliver social justice” will be theirs.

The propaganda of emotional blackmailers is already everywhere. If you don’t vote for Corbyn, you hate the poor. If you don’t vote for Johnson, you love the suicide bombers. I want to emphasise instead how hard it is to persuade voters to defend their society by switching their votes. We have two possible futures: one that contains a fleeting flash of hope and a second that is thoroughly miserable but is no more than we deserve.

Pacts against extremists can be organised, as long as there is only one of them. Twice this century, the French left has swallowed all its principles except the most important principle of all: the need to stop the far right seizing control of the republic. In the 2002 presidential election, under the stirring slogan “Vote for a crook, not for a fascist”, it supported the conservative Jacques Chirac and stopped Jean-Marie Le Pen, leader of the extreme-right Front National, becoming president. In 2017, the left backed Emmanuel Macron against Marine Le Pen. Anyone who finds comparisons with the French far right offensive should consider that Ms Le Pen has watched the Brexit disaster that Johnson and Nigel Farage brought and concluded that advocating French withdrawal from the EU was too extreme a policy for even the French far right to endorse. Meanwhile, Corbyn has engaged with antisemites as gruesome as any the Front National supported.

The conservatives of the #NeverTrump movement tried to imitate the French left in the 2016 US presidential elections. Trump was a pornographic slob, a sociopath and a clear and present danger to the US constitution, they warned. Every word was true. The logic of their argument was that moderate conservatives had to vote for Hillary Clinton. They didn’t in sufficient numbers, for a reason that bears on the British crisis. However absurd their judgment appeared to outsiders, too many conservatives regarded Clinton as just as extreme as Trump.

If enough Liberal Democrats, Greens and nationalists win, they can limit Johnson or Corbyn’s worst excesses

Once that view has taken hold, people cling to their tribe, however ugly and dangerous its leader is. This is Johnson and Corbyn’s best hope. They want their nervous supporters to be scared into voting for them for fear of the other.

Britain does not face a binary choice. After Johnson’s purge, the Tory benches will be full of hard-faced hacks determined to impose Brexit, whatever the cost. Momentum will ensure the next batch of Labour MPs will contain more incontinent cranks than any country deserves. We are about to elect a parliament of freaks and fanatics. If, however, enough Lib Dems, Greens and nationalists win, they can act as a blocking minority that will limit Johnson or Corbyn’s worst excesses.

But there I go, playing fantasy politics. The fate of the #NeverTrump movement shows the likely fate of Britain. David Frum, one of its doughtiest spokesmen, tells me the experience of Trump in power is giving it a belated vindication. In the midterms and last week’s elections in Pennsylvania and Virginia, it was not just young, educated voters and minorities who voted against Trump but wealthy suburbs that remain in many other respects conservative.

On this reading, Britain will have to experience a Johnson Brexit or a Corbyn government before enough voters turn against them. It is as if large sections of the population have reverted to childhood and must learn all over again that there are no fairies in our #NeverNeverLand, only monsters.

• Nick Cohen is an Observer columnist