The Liberal Democrats, the Green Party and Plaid Cymru have made an electoral pact under the rubric ‘Unite to Remain’. The three parties have agreed to stand down candidates in sixty constituencies: in 43, only a Liberal Democrat will stand, ten will have only a Green candidate, and seven only a candidate from Plaid. The purported aim is to return the maximum possible number of pro-Remain MPs to parliament; the parties’ self-interest is an unspoken factor. The lash-up may help Plaid and the Greens win one or two target seats, but the chief beneficiaries – if it succeeds – will be the Lib Dems, who are looking to retake a slew of Tory-held seats in the south of England.

Electoral pacts are rare in British politics: they tend to follow periods of profound national crisis, as in 1918 or 1931. Brexit has cut across party lines, fractured the constitution and divided the country, but it isn’t on the scale of the First World War or the Great Depression. Recent drives for an anti-Tory ‘progressive alliance’ have been distinctly minoritarian affairs, foundering on long-established ideological hostility between parties, disincentives for dominant parties to concede advantage under first-past-the-post, and Labour’s deep-rooted understanding of itself as the only ‘progressive alliance’ that really matters.

The Unite to Remain pact does not include the two major parties committed to a second referendum, Labour and the SNP. Without them, there is no realistic path to the UK’s remaining in the EU. In some of the seats already announced, a pact candidate will be standing against a pro-Remain Labour MP. In Bristol West, for example, the Greens’ Carla Denyer will stand against Labour’s vocal Remainer Thangam Debbonaire. Her majority increased substantially in the last election, from under 6000 to more than 37,000, but is likely to suffer from an aggressive Green campaign.

In tight Labour/Tory marginals with incumbent Labour Remainers, the pact’s tactics seem even more questionable. The close race in Stroud between David Drew – another pro-Remain Labour incumbent – and the Tory challenger may depend on how many voters swing to the pact’s Green candidate, Molly Scott Cato. Such a vote there looks like a vanity exercise, and risks returning a Tory MP signed up to Johnson’s Brexit deal.

Unite to Remain’s strategy is based on two questionable assumptions: that only the parties signed up to its agreement qualify as sufficiently ‘Remain’, and that the election turns solely on the issue of Brexit. For all the Lib Dems’ excitement at the prospect of Jo Swinson walking into Downing Street in December, any government formed at the end of this year will be led by either Labour or the Conservatives, and only Labour offers Remainers the hope of a second referendum. It’s peculiar that this should have vanished from the strategic calculus. One reason is an optimistic reading of the European elections, when the Lib Dems did well (on a turnout of 36 per cent); another is Swinson’s need to woo soft Tory voters in her target marginals. But the fumes of remainer conspiracy theory also waft around the decision: the Lib Dem president, Sal Brinton, said that the party would have worked with Labour but ‘Corbyn stands for Brexit’ – even though he is explicitly standing for another referendum.

Liberal Democrats grow especially fork-tongued during election campaigns (the mendacity of their campaign leaflets is proverbial) but Brinton’s other reason is more compelling: Labour will not stand aside for Lib Dem candidates. Few in the Labour Party have forgotten or forgiven the Cameron-Clegg coalition; they argue, too, that the politics of a general election ought not to turn on a single issue, even one as vital as Brexit. The Lib Dems’ rightward drift makes any rapprochement politically improbable.

For those on the left of the Labour Party, who have recently celebrated the party’s adoption of a Green New Deal, there is bafflement at the Greens’ willingness to overlook other political differences to pursue a marriage of convenience with the Liberal Democrats on Brexit. Labour, like the Green Party, aims for net-zero carbon emissions by 2030; the Lib Dems’ target is 2045. The party has repeatedly voted against the regulation of fracking; Swinson took a personal donation of £10,000 from Mark Petterson, the director of a fracking company, in 2017, and a further £4000 last year.

Some Greens are suspicious of their new bedfellows: after Swinson castigated Jeremy Corbyn for being insufficiently enthusiastic about nuclear holocaust earlier this week, the recently selected Green candidate in Bermondsey and Old Southwark tweeted ‘Fuck the lib dems’; a few hours later she stood down in favour of her Lib Dem rival. Green voters in that constituency, who tend to swing left, are now faced with a choice between a candidate on the right of a party with a left-wing, environmentally ambitious policy programme, and a Liberal Democrat. All votes are always tactical.