Like it or not, who leads the Lib Dems matters: Labour can't win without them

Don’t turn away, but the UK’s most boring leadership election ends on 26 August. Few but their closest coteries will be waiting with bated breath to learn which of two similars has won the dubious prize to lead a group of 11 straggling Liberal Democrat MPs in the House of Commons. (Wait, apologies: the non-election, last week, of the only candidate to lead the beleaguered Tories in Scotland is possibly an even duller event.)

You can hear Labour people, between gritted teeth, asking loudly why the Lib Dems don’t just do the decent thing – go away and expire. Leave the stage. What is their point, after a hundred years of failure? Like loose horses in the Grand National, they’re an annoyance in a two-party race. Lib Dem politics is for hobbyists who love elections but lack purpose. They sweep up ditherers’ wasted votes and lack the nerve to be red or blue in a blood-feud political battleground. Besides, let them never be called progressive again after the abominations they enabled in the austerity coalition. The bedroom tax, anyone?

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Right, Labour people, once you have vented all that, take a deep breath and think again. The Lib Dems matter. The bitter truth is that it is virtually impossible for Labour to win in 2024 unless the Lib Dems do very well. Suck it up, but you have to wish them well and speed them on their way.

I watched one of the scores of hustings showcasing Ed Davey, the acting Lib Dem leader, and the 2017 Commons entrant Layla Moran. Both put out thoroughly decent pitches, and came across as nice people with nice politics, for a fairer, greener country. Moran pitches to students: her hands are clean on the great Lib Dem tuition fees betrayal, she calls herself “more radical than Labour”. (No politician is not “radical” these days.) She identifies with the left-leaning strand of the late Charles Kennedy.

Ed Davey, who has more than 20 years of Commons experience, stresses the hard graft, “no quick fixes” task ahead, and is more a Paddy Ashdown type. He talks up his commitment to a caring society after a childhood as his mother’s carer: he plugs how as energy minister he trebled renewables against Tory instincts. To an outsider, only nanoparticles separate them. Neither would ever back the Tories, or not under Boris Johnson. No coalitions: they learned the hardest way when it wiped them out, costing 49 seats in 2015.

The bookies back Davey, with the solid support of almost every Lib Dem they’ve heard of. Moran has the advantage of newness, untainted by the party’s austerity past. But being a woman may not help after Jo Swinson’s cataclysmic 2019 election, where she seized spectacular failure from what should have been a historic opportunity. Voters were faced with an epically nightmarish choice between Johnson and Jeremy Corbyn, when only the Lib Dems had a crystal-clear pro-EU pitch to millions of distraught remainers. Why the wipe-out?

To be sure, Swinson ran a dismal campaign, but that wasn’t enough to sink them by itself. Counterintuitively, it was Jeremy Corbyn’s extreme unpopularity and Labour’s toxic brand that capsized the Lib Dems. If Corbynites notch that as up as a triumph, beware: Labour and the Liberal Democrats sink or swim together in current politics. Here’s why.

Lib Dems beat Tories in places where Labour haven’t got a prayer: 80 out of the 91 seats where the Lib Dems came second are Tory, as are more than 40 seats where Lib Dems scored over 30%. In Labour-Tory marginals, a high Lib Dem vote takes many more votes from the right, according to political scientist Rob Ford. In an arc of leafy seats around London, the Lib Dems could have scooped up soft Tory remainers, appalled by Johnson. What stopped them, says Ford, was fear of Corbyn. “Keir Starmer doesn’t frighten the horses with those voters: they could risk voting Lib Dem next time.” Lib Dem success aided Labour in Tony Blair’s 1997 landslide, “but Labour starts much further back now, needing a 12-13% swing, with lost Scotland seats, so the Lib Dems will be more important”. Labour votes are even more densely concentrated in metropolitan and university seats than they were: Ford’s home, in the Manchester Withington constituency, has a 56% Labour majority.

Related: Lib Dem leadership hopeful Layla Moran hints at closer links with Labour

No one talks of coalition, but the unshakeable condition for the Lib Dems to support a minority Labour government would be proportional representation in the first Queen’s speech. They won’t make Nick Clegg’s idiotic mistake of accepting a referendum on a terrible system like the alternative vote, torpedoed with such success in 2011. Starmer has backed a constitutional convention and electoral reform, but will need to push it through some diehard dinosaur recalcitrants to get it into the Labour manifesto. Nonetheless, a YouGov poll shows 75% of Labour members back PR.

The paradox is that Labour and the Lib Dems need each other, but must turn their backs on each other. No pacts, no cooperation – keep up the tribal hissing and spitting, says Ford, but Neal Lawson of Compass reminds Labour how vital it is for both parties to direct their electoral energies away from seats the other can best win.

Before they can do this, Labour still needs to escape its own toxicities: the looming hulk of Len McCluskey shakes Unite’s moneybags menacingly against the new leadership, while Corbynite former officers threaten pointless legal action. Some Corbyn supporters still deny the party has a problem with antisemitism, while blaming Jews for conspiring against Corbyn.

The latest YouGov polling puts Starmer ahead of Johnson on their personal ratings – but Labour is still six points behind. Asked “Is Labour ready to form the next government?” 25% say yes, 51% still no. Labour has time to convince voters: Starmer and his earnest front bench are only in the foothills of the mountain they have to climb. It doesn’t matter who wins the Lib Dem leadership, but their fate relies on Starmer’s success – and vice versa.

• Polly Toynbee is a Guardian columnist