The Labour leadership vote is at last under way. Let no future contest hold such a purgatory of hustings, forcing candidates to say something new every day: Lisa Nandy and Rebecca Long-Bailey fell into the trans thorn bushes, while Keir Starmer may have overpromised.

Outsiders, puzzled by the contenders’ misleadingly similar pitches, should remember what a peculiar electorate they are addressing. Belonging to a party is a niche hobby: the tiny electorate of white male shire Brexiteers who chose Boris Johnson were the oddest. But YouGov revealed Labour members’ eccentricities. Asked for their most admired leaders, Corbyn topped the poll at 71%, with Tony Blair, their three times victor, trailing at 37%. When Rebecca Long-Bailey awarded Corbyn 10 out of 10 for leadership, she knew her voters.

The party’s members are weighing up which qualities will matter most in 2024: solid principles, strategic cunning, endurance

Nonetheless, YouGov also suggested a Starmer win by 63% to 37% if there was a final run-off against Long-Bailey. Labour members may yearn for everything the 2019 manifesto promised, with no painful choice of priorities, but after four miserable defeats they are ready to bend. Only 14% would make no compromises on Labour values to make the party more electable.

But how would they feel about the huge compromise that has just loomed over the horizon, proposed by two people some Labour members may be disinclined to hear – Tony Blair and Liberal Democrat leadership candidate Ed Davey? In Blair’s sweeping – and brutal – tour of Labour’s history at a speech marking the party’s 120th anniversary, he warned of a “make-or-break” crisis. “The Labour party is not an NGO, and not a pressure group. Its aim is not to trend on Twitter, or to have celebrities (temporarily) fawn over it … Our task is to win power.”

Listing electoral failures, he blamed a historical structural fault: that first split with the Liberals. “We must correct the defect from birth, which separated the Liberal reforming traditions of Lloyd George, Beveridge and Keynes from the Labour ones of Keir Hardie, Attlee, Bevin and Bevan.” He called for a “whole new progressive alliance” to bring back these artificially separated strands.

Frankly, the notion of a merger is for the birds. Although many Labour members advocated tactical voting for Lib Dems in unwinnable seats, that’s a million miles from permanent union – Lib Dems stay unforgiven for coalition austerity. Nor would their members take kindly to mixing their DNA with a Corbynised party. But Davey gingerly put a toe in that water. Picking up Blair’s challenge in the Sunday Times, he wrote: “Facing this rightwing Conservative administration, progressive politicians must at least explore how we might work better together, even tentatively.”

Lisa Nandy, Rebecca Long-Bailey and Keir Starmer at a Labour party leadership hustings in Durham. Photograph: Ian Forsyth/Getty Images

Mid-election, Labour candidates may shy away from such treason. But the Liberals have always lain like a water buffalo across Labour’s tracks, as Roy Jenkins once put it. First-past-the-post allows no third party, except as a wrecker, and it has split the non-Tory vote time and again. But the last thing our decaying democracy needs is less choice. What’s needed is a one-time-only pact to not stand against each other in any seat, with a manifesto promise to bring in proportional representation as a first act, needing no referendum.

Looking across the Atlantic we see the worst of first-past-the-post two-party constriction, forcing into one bed Bernie Sanders and Michael Bloomberg, making traditional Republicans cohabit with Donald Trump. Under a civilised voting system, the Tories would have split between pro- and anti-Europeans – and there would be no Brexit.

There should be a socialist and a social democratic party. Trying to contain both has led to historical Labour strife. Talking to Labour members on a phonebank this weekend, I heard many threaten to walk if Long-Bailey wins, while some said they’d quit if Starmer showed any sign of turning “Blairite”.

Four years is a long stretch to the next election, with Brexit and Johnson fallout as yet unfathomable. Labour members are weighing up which qualities will matter most by then: endurance, a rhinoceros hide against all attacks, solid principles and strategic cunning. Some demand charisma, but that may be a fading asset after years of Johnson japes. In my view, a Labour party led by a serious heavyweight former prosecutor, backed by a strong new shadow team, would be a formidable opposition to Johnson chaos and his low-calibre cabinet. To beat a majority of 80 in one election is an Everest to climb. By that time, Labour and the Lib Dems may be ready to make a deal to block another Tory victory.

They could usher in an electoral multiparty system fit for a modern pluralist democracy. No more dragooning electors to the ballot box, holding their nose to vote for the least loathed of two portmanteau parties. Nandy says she will consider reform. Starmer says “there’s a case for reform, because of the millions of votes wasted every election in safe seats”. Make Votes Matter reports that three-quarters of Labour members back proportional representation. This is hardly surprising, since it took an average of 51,000 people to elect a Labour MP in December, only 38,000 for each Tory. The public backs PR, with 56% saying seats won should match votes cast.

The next few years will be a gruelling uphill slog, watching the Tories do things while Labour can only say things from the sidelines. The party should listen to Blair and Davey: a temporary pact may be the best chance of preventing another slaughter at the polls. A new leader can open new doors.

• Polly Toynbee is a Guardian columnist