The Liberal Democrats, the Green Party and Plaid Cymru have signed up to an electoral pact which could make life harder for the Conservatives in several constituencies on election day.
All three parties share pro-European platforms and have agreed a deal not to field candidates in 60 constituencies where they are up against Conservative MPs or challengers.
The plan could be bad news for Prime Minister Boris Johnson as it might deprive him of the majority he could expect if the opinion polls are borne out on 12 December.
It could also be bad news for Foreign Secretary Dominic Raab after the Green Party agreed not to field a candidate in his seat, Esher and Walton, in the hope of giving the Liberal Democrats’ Monica Harding a better chance of defeating him.
Last time round the Lib Dems were third but they expect to overtake Labour in a constituency which is largely pro-Remain.
The Greens only got 1,000 votes in Esher - which is on the outskirts of London - last time round but if the Tory vote collapses, or some of it splits off to vote for the Brexit Party, they could be enough to turn it into a marginal.
The Conservatives, whose campaign slogan is “Get Brexit Done”, are facing concerted opposition from Plaid Cymru, the Lib Dems - who are traditionally strong in Wales, the west of England and parts of London and the south east - and the Greens, who launched their campaign on Wednesday, 6 November, and are hoping to make climate change the biggest issue.
Heidi Allen, a former Conservative MP who joined the Liberal Democrats but then decided against trying to defend her Cambridgeshire seat, said it was the first national electoral pact in Britain since the Liberals and the SDP tried it in the 1980s.Ms Allen, a devout Remainer, said the Lib Dems "know this general election is the last chance to change the path that this country is currently on.”
The pact should boost the Liberal Democrats’ chances of winning in Cheltenham, Winchester and Montgomeryshire while Plaid Cymru hope it will help them win Ynys Mon (Anglesey), where the Labour MP Albert Owen - who won the seat from Plaid Cymru in 2001 - has stood down.
Ten of the constituencies involved in the pact are in Wales where the Welsh nationalist Plaid Cymru have teamed up with the Lib Dems and Greens.
But one Welsh seat which is not included in the deal is Ceredigion, where the Tories trailed in fourth place in 2017. The Plaid Cymru MP Ben Lake, who pipped the Liberal Democrat Mark Williams by 107 votes last time, will have to do battle with him again.
Mr Williams won the seat from Plaid Cymru in 2005 but lost it in 2017, largely because student voters at Aberystwyth University switched to Labour from the Lib Dems.
At a by-election earlier this year the Liberal Democrats seized Brecon from the Conservatives after Plaid Cymru and the Greens opted not to run candidates.
The Scottish National Party (SNP) - which is also vehemently against Brexit - has opted to stay out of the electoral pact, because it believes it can gain four Liberal Democrat seats in Scotland - Edinburgh West, Orkney & Shetland, Caithness and East Dunbartonshire, where the Lib Dem leader Jo Swinson is the MP.
Labour is also having nothing to do with the Remain pact because it believes it could hurt it in Labour-leaning constituencies which voted to Leave in 2016.
Whereas the Liberal Democrats have said they will cancel Brexit, Labour’s Jeremy Corbyn is adamant he wants to respect the result of the referendum but simply wants to renegotiate the deal to improve workers’ rights and environmental protection.
Labour's Shadow Chancellor, John McDonnell, poured scorn on the pact at a rally in Liverpool on Thursday, 7 November.
He said: "We are going to have a majority Labour government - we don't need any pacts. We will never enter pacts, coalitions, or deals like that - ever.”
That will come as a blow to Labour’s candidate in Southampton Itchen, Simon Letts, who is trying to overturn a Tory majority of 31. The Greens got 725 votes in 2017 and if they field a candidate this year it could deprive Labour of victory.