The Tory electoral strategy is straightforward: unite leave voters behind the Conservative banner. Nigel Farage’s decision to form a de facto pact with the Tories should serve as a moment of clarity. The differences between Farage and Boris Johnson, Donald Trump’s two principal British allies, are merely personal: politically, they are on the same page. Both fundamentally see Brexit as a blunt instrument to reshape British society, stripping away the pesky workers’ rights and consumer and social protections that stand in the way of their hyper-Thatcherite dystopia.

John Major once declared that the NHS was as safe with Johnson and Michael Gove “as a pet hamster would be with a hungry python”; Farage is on record supporting its privatisation. This pact should be treated as a national political emergency: in four weeks, it may triumph and transform Britain for a generation.

Labour’s best hope is to mobilise its key strength – its mass membership

The second plank of the Tory strategy relies on the Liberal Democrats dividing the remain vote. Jo Swinson may have valid reasons, on her own political terms, for waging her party’s rampantly anti-Labour campaign. The Lib Dems’ only real chance of picking up significant numbers of seats is in the south-west, and that means winning over Tory remainers who abhor the very notion of a Corbyn-led government.

By focusing her attacks on Labour rather than the Tory Brexiteers, Swinson believes she will reassure those David Cameron-type supporters that a vote for the Lib Dems will not install Corbyn in Downing Street. It is why Swinson so aggressively opposed a temporary Corbyn premiership to extend article 50 and call an election, and why she refuses to stand down candidates against anti-Brexit Labour MPs.

The problem is that the future of the country is rather more important than whether the Lib Dems win back St Ives, Cheltenham or North Devon. Canterbury was a Tory fortress from the mid-19th century until Labour’s Rosie Duffield triumphed in 2017 with a majority of just 187 votes. The Lib Dems’ Tim Walker stood down for the principled, commonsense reason that if he succeeded in dividing the anti-Tory vote, the hard Brexit Tory candidate – a Vote Leave veteran – would take the seat instead.

The Lib Dem leadership’s response? Disciplining Walker and imposing a new candidate instead. That led a Lib Dem candidate, Guy Kiddey, in the marginal of High Peak – held by Labour with a majority of 2,322 – to threaten to resign from the party. He was later told he would be replaced as the Lib Dem candidate.

The Lib Dems will protest that this is terribly unfair. Are they not a national party with the right to stand wherever they choose? Yes, this is how politics works. But the problem is this: the party has made stopping Brexit its defining cause, while simultaneously portraying it as a crusade that transcends party politics. The Lib Dems know that throwing resources at seats they cannot win and peeling away significant numbers of Labour voters will allow the Tories to win. They know that demonising Labour as a “Brexit party” – when a Labour-led government implementing its policy of a second referendum is the only plausible route to stop Brexit – divides remainers to the benefit of the Tory Brexiteers. They know that focusing their vitriol on Corbyn strengthens the position of Johnson.

But ultimately they are not, by definition, an anti-Brexit party; the remain cause is secondary to increasing the number of Lib Dem seats in parliament. If your main aim in politics is to advance the partisan interests of the Lib Dems and repeat their performance in government, then this makes sense. If your only cause is stopping Brexit, however, it does not.

There will be some who vote Lib Dem in four weeks’ time because of dishonest graphs printed on leaflets claiming that only Swinson’s party can defeat Johnson in tight Tory-Labour marginals. If they wake up to see a beaming Boris Johnson, relishing his newly minted Tory majority and pledging to implement a Trumpian hard Brexit by the end of January, they will feel horrified and tricked.

That does not absolve Labour of its responsibilities. No party has a divine right to any voters. In 19 November’s televised debate with Johnson, Corbyn must surely make it clear to disillusioned remainers that Labour’s referendum policy is their only chance of stopping Johnson’s Brexit.

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This doesn’t mean a return to the cynical “you might hate us but the Tories are worse” politics of the New Labour years – in fact, the party’s commitments to hiking tax on the top 5% to invest in public services, public ownership and scrapping tuition fees are very popular. A tightrope has to be trodden, of course: Labour supporters who voted for Brexit, particularly in leave-leaning northern constituencies, cannot simply be ignored.

The party’s challenges go beyond Brexit. A new poll from Survation finds that just 45% of 18- to 34-year-olds and less than half of those earning under £20,000 are certain to vote, compared with 81.5% of over 65-year-olds and 77% of those earning more than £40,000. It is these people – who should be natural Labour voters – that the party urgently needs to get out.

Labour’s best hope is to mobilise its key strength – its mass membership – and focus on its popular domestic policies and a positive vision. That does not mean avoiding the obvious truth, however, which is that the Tory Brexiteers’ best hopes rely on the Lib Dems. If a Tory hard Brexit is unleashed in the coming weeks, then Jo Swinson and her cynical Lib Dem strategists will rank among its chief handmaidens.

• Owen Jones is a Guardian columnist