An alliance between progressive parties would be fragile, but it could win an election

A general election is now more or less certain, the only remaining question is, “How soon?” We know where we want to go but we’d rather not be starting from here. If the aim is not just to deny Boris Johnson a majority, but to get rid of the Conservatives as the party of government altogether, the best scenario would be Labour polling high, having created a solid anti-Brexit, anti-hard right, anti-austerity tripod.

That hasn’t happened, but a progressive alliance is still more than capable of having a decisive impact. Nigel Farage says a pact between his Brexit party and the Tories would make them “unstoppable”. Yet the focus on Johnson’s ability to ally with Farage is misplaced. The Conservatives and the Brexit party will certainly pull together; the only unknown is how many core Tory values the prime minister is prepared to traduce to get there, which is hardly unknown at all.

A progressive alliance, on the other hand, would be much more fragile and its conclusions less foregone. The fundamental block is the Labour party – obsessed with vote share, it has always opposed alliances of any kind. In 2017, if it had stood aside in the Isle of Wight – bear in mind that this seat has never returned a Labour MP – the Greens would have repaid them in 12 other constituencies. But no dice, then, and advisers around Jeremy Corbyn were scornful about the successful alliances made between the Greens and the Liberal Democrats, on the basis that their main impact had been to reduce the Green vote share (cut by half on the 2015 election).

While Labour frontbenchers talk constantly to Liberal Democrats – indeed, last week’s parliamentary victories couldn’t have been won without a fair amount of talking to Tories – nobody at shadow cabinet level is talking about pacts. Emily Thornberry can just about hear the word “alliance” without recoiling, but even she – pragmatic, in a 70% remain constituency – is not practically seeking one.

The Lib Dems have erected their own barriers, mainly in the form of their performative hostility towards Labour. Their full revoke stance, adopted this week, is not a meaningful contribution to the Brexit process, unless they believe they’ll win an election outright – in which case they have taken leave of their senses. Rather, it’s designed to underscore their superiority over the Labour party, in the eyes of hard remainers; entirely speciously, since they know they’ll never be called out on it.

Taken together with the much friendlier noises they make to Tory defectors than to anyone in the Labour hierarchy, it is hard to perceive much warmth between the two blocs. However, this is mostly strategic; it will help both the Lib Dems and Labour if remainer Tories – has any group ever been so politically homeless? – find somewhere to cast their votes, otherwise that’s a 4 million-strong army of silence. It would be much harder for them to vote for Jo Swinson’s party were it seen to be chummy with Corbyn’s, and the smarter elements of Labour take their catcalls with a pinch of salt.

A large, formal pact between the major remain forces – Labour, the Lib Dems, the Greens, Plaid Cymru and the SNP – is off the table. A month ago, people were talking about 1918, when the Liberals and Conservative coalition won a landslide victory by fielding a single candidate for both parties on a seat-by-seat basis. Never such innocence again.

But the jeopardy of a far-right Tory majority has introduced a trace of humility (“With the real prospect of an election, people begin to understand their real size,” one pollster told me. “So Anna Soubry acts like she’s Germany, but realises she’s actually Luxembourg”). According to Best for Britain data, around 457 seats are immune to alliances, leaving around 150 marginals. In 111 of these, the numbers strongly implore Lib Dems, Greens and Plaid to swing behind Labour (in 30 seats, even a 5% shift from Labour to the Lib Dems would deliver the seat to the Conservatives).

This is well understood by parliamentary candidates, who are in constant talks about non-aggression pacts. Seats such as Portsmouth South, Battersea, Bermondsey and Old Southwark, and Kensington and Chelsea are tantalising for the Lib Dems – either because they’re the strongest second party and could leverage their bolder Brexit position, or because they’ve had a recent surge – but the best bet for a progressive MP in those seats is for everyone to campaign for Labour. There are other seats where the Liberals have to run strong campaigns and be seen to do so. Hope Not Hate has data on Justine Greening’s seat, Putney. The Labour candidate, Fleur Anderson, is strong and well liked, but the numbers – Tories 34%, Lab 22%, Lib Dems 30%, with a 7% Green showing – mean that anyone putting together tactical voting cribsheets would go Lib Dem.

Straight tactical voting could work beautifully, or it could go very badly: if lots of different campaigns with different organisational interests make different recommendations, that will be wasted energy and, ultimately, wasted votes.

Non-aggression pacts across clusters of seats are always under-reported, since they happen at grassroots level. But they’re vital – the Greens in Norwich South would not have swung behind Labour’s Clive Lewis in 2017 if the red-green candidates in neighbouring constituencies had been savaging each other. In the same year in East Sussex, there was an explicit trade in seats between the Lib Dems and the Greens, which nearly pulled off a progressive republic in a sea of blue.

In Cumbria, too, activists negotiated under the radar across five seats; and there was “quite a lot of speed dating going on between the parties”, as one Lib Dem activist put it, in Frome, Bristol West, Gloucester and Stroud. This is very similar to the informal alliances of 1997, which is the interesting bit; you expect a progressive alliance to look scrappy, patchworked, cobbled-together. But when it works, it often looks more like a landslide.

• Zoe Williams is a Guardian columnist