The electoral calculations behind Boris Johnson’s Brexit policy are clear. If he can unite Leave supporters behind the Conservative party while the Remain vote is scattered he will secure the foundations for winning an overall Commons majority. This strategy has already had some success: support for the Brexit Party has fallen by around six points during the summer, with more or less all of it switching to the Conservatives. On average the polls suggest that nearly half of Leave voters would back Boris Johnson in an early ballot.

In contrast the Remain vote is divided between Labour (34 per cent) and the Liberal Democrats (29 per cent) while the SNP dominate among pro-EU voters north of the border. As a result, although the Conservatives’ average overall poll standing is no more than 32 per cent, it is enough to give them a seven-point lead over Labour, and still leaves them well ahead of the Liberal Democrats on 18 per cent. Meanwhile, the Brexit Party has been pushed back to no more than the 13 per cent that Ukip won in 2015.

True, thanks to revival in Liberal Democrat fortunes and the strength of the SNP, that seven-point lead over Labour could still be insufficient to produce a Tory majority. Indeed, the party could well be left with more or less the 318 seats that Theresa May won two years ago. But assuming Johnson could retain the support of the DUP, MPs from the parties in favour of a second EU referendum would still be outnumbered by pro-Brexit supporters.

Meanwhile, polls suggest that if the prime minister succeeds in delivering Brexit, around half of the remaining Brexit Party vote might also switch to the Tories, thereby giving them a healthy overall majority.

Cliffs of Dover lit up in Brexit protest Show all 5 1 /5 Cliffs of Dover lit up in Brexit protest Cliffs of Dover lit up in Brexit protest Campaign group Led By Donkeys projected this statement by Nigel Farage on the Cliffs of Dover on the evening of April 4 @ByDonkeys / Twitter Cliffs of Dover lit up in Brexit protest Campaign group Led By Donkeys projected this statement by former Brexit secretary Dominic Raab on the Cliffs of Dover on the evening of April 4 @ByDonkeys / Twitter Cliffs of Dover lit up in Brexit protest Campaign group Led By Donkeys projected this message to the EU on the Cliffs of Dover on the evening of April 4 @ByDonkeys / Twitter Cliffs of Dover lit up in Brexit protest Campaign group Led By Donkeys projected this message on the Cliffs of Dover on the evening of April 4 @ByDonkeys / Twitter Cliffs of Dover lit up in Brexit protest Campaign group Led By Donkeys projected this message on the Cliffs of Dover on the evening of April 4 @ByDonkeys / Twitter

A united Leave vote will have outgunned a divided Remain vote, even if, as polls suggest, these days Remain supporters somewhat outnumber their Leave counterparts. So perhaps those on the Remain side of the Brexit debate should find a way of uniting their divided forces?

They might hope that Remain voters will do this for them, voting tactically for whichever party is best placed locally to defeat the Conservatives. There were some signs of that in 2017. The Liberal Democrats advanced at Labour’s expense in the limited number of Conservative-held constituencies where they were still a credible challenger locally. However, if that pattern were replicated this time it might do no more than switch half a dozen or so seats from the Conservatives to the Liberal Democrats. That might be enough to make Boris Johnson’s life uncomfortable in the absence of a further squeeze by the Tories on the Brexit Party vote but it could still be insufficient to ensure a pro-second referendum majority.

The Liberal Democrats are too far behind in most seats for tactical voting alone to bring about a transformation in their fortunes. Labour, in contrast, are neck and neck with the Tories in many a seat. Tactical switching by Liberal Democrat voters in favour of Labour could make a significant difference, especially in seats such as Kensington and Canterbury that Labour currently hold by only a small majority. A five-point tactical switch to Labour in such seats could cost the Conservatives at least 20 seats and create a pro-second referendum majority. But whether that many Liberal Democrat voters would be willing to make such a switch is far from clear. Labour would have to convince them that the Conservatives could win locally and that a vote for Jeremy Corbyn would indeed help deliver a second EU referendum.

Maybe, therefore, Remain-inclined politicians should try to unite the Remain vote themselves by forming an electoral pact? This, after all, is what happened in the recent Brecon by-election, when both Plaid Cymru and the Greens stood down in favour of the Liberal Democrats, a decision that may well have been crucial in delivering a Liberal Democrat victory. However, a pact between just these three parties in which only the best placed of them stood in any Conservative-held seat in England or Wales would only likely make a difference in a handful of seats. It is far from being a sure-fire way of ensuring Boris Johnson’s defeat.

Meanwhile, given the intensity of the debate about Scottish independence, it seems unlikely that it would be possible to form any pact involving the SNP. In any event, the nationalists would seem well set to claim most of the spoils north of the border anyway. What, however, could potentially make a difference is a pact that involved Labour.

Even if we take into account the likelihood that some of their supporters would stay at home or spoil their ballot, a pact in which the Liberal Democrats, Green and Plaid Cymru stood down in marginal seats that Labour was defending against a Conservative challenge or was trying to wrest away from the Conservatives could potentially cost Boris Johnson up to 50 seats, most of them to Labour. But for that to happen Labour would have to be willing to stand down in seats where it had little chance of winning – a step that hitherto the party has always been reluctant to take.

How could a no-deal Brexit be stopped?

Meanwhile, the Liberal Democrats, the Greens and Plaid Cymru would have to ask themselves whether they would be willing to form a pact from which Labour would most likely be by far the principal beneficiary and which might help propel Jeremy Corbyn into Downing Street.

Overcoming the party divisions among Remain politicians may well prove to be not that easy.