Areliable test of a party’s campaign readiness is the pub justification question. Imagine friends having a drink on the eve of polling. Like most people in the country, they usually avoid talking politics. They don’t want their preferences interrogated. The more efficiently they can justify a choice to themselves and their mates, the better. Why vote Tory in 2019? Get Brexit done. Three words. The Labour offer was trickier to summarise. Many of the party’s own MPs couldn’t explain why Jeremy Corbyn should be prime minister. And now? Why vote Conservative in 2024? Downing Street would like the answer to be economic stability, tax cuts and stopping the boats. But after four terms in office, the Tories have delivered recession, a record high tax burden, and the boats keep coming. Plus, public services are falling apart. That record does the opposition’s pub justification for them. Why Labour? Get the Tories out. Last week’s byelections in Wellingborough and Kingswood confirm the trend: a hefty swing to Labour, amplified by Liberal Democrats voting tactically and Tory loyalists staying at home. Repeated nationwide, those conditions would bury Rishi Sunak’s party in a landslide. It wouldn’t be an enthusiastic endorsement of Labour, which makes the party’s more committed supporters uncomfortable. It would be easier to envisage Keir Starmer thriving in office if he surfed into No 10 on a wave of popular goodwill. Instead, he will be deposited there as jetsam from an anti-Tory tide. On the upside, low expectations are easy to surpass. Starmer is no mere passenger on the swinging pendulum. He has made himself strategically inoffensive to voters who will only switch from the Tories once reassured that Labour isn’t a cult for unpatriotic, spendthrift cranks. There is nothing unusual about victories secured by what political science calls “negative partisanship” – voting with intent to block the least-desired candidate. (Hostility to Corbyn was at least as big a factor as Brexit in 2019.) It is rational to play that game when the electoral system demands it. Sunak’s response to the loss of two MPs last week had nothing to do with his own party’s merits and everything to do with first past the post. The prime minister warned wavering Conservatives who are flirting with Reform UK that they were effectively voting “to put Keir Starmer in power”. Downing Street’s plan to avert catastrophe depends on wooing disillusioned Tories who are getting a taste for Reform as the most pungent available brew of social illiberalism and hostility to immigration. Those voters don’t like Starmer. But there isn’t a lot of evidence that they fear Labour’s ascendancy more than they want to punish Conservatives, whose betrayals feel more salient. Meanwhile, a policy of hugging Reform voters tight involves Sunak turning his back on moderate, centrist Tories. They have already had enough of spiky rightwing populism and are seeking solace with the Lib Dems. An edifice that, four years ago, looked like a broad church of Brexit conservatism under the charismatic ministry of Boris Johnson turns out to be a temporary warehouse for disparate voters, straddling unstable ideological faultlines. This, too, is a function of an electoral system that obstructs new entrants and penalises small players. It masks underlying volatility. The Labour-Tory duopoly has been in slow decline since the 1950s, when the two big parties regularly took a combined vote share of more than 90%. It was 75% in 2019. British voters flee red and blue banners whenever offered a more proportional ballot. The Holyrood voting system gave Scottish nationalists the breakthrough they needed to become a party of power. Senedd elections have given Plaid Cymru a purchase on Welsh politics. In 2019 elections to the European parliament, the Tories came fifth, behind the Brexit party, the Lib Dems, Labour and the Greens. That result expressed a peak of misalignment between Brexit politics and traditional patterns of party support. But the unusual circumstances don’t prove that the fragmentation was superficial. It could be that the schisms were profound and the subsequent reassertion of Labour-Tory dominance is a fragile artefact of first past the post. Sunak’s current woes derive from his failure to pretend that two different entities – nostalgists for a pre-Brexit, liberal centre-right politics, and a post-Brexit populist front – can both be called the Conservative party. Starmer has been more successful in papering over equivalent fissures on the left. “Get the Tories out” is powerful glue. But the adhesive dissolves once the Tories are actually out and the ugly compromises of government begin. Labour leaders can’t expect prolonged gratitude for victory once the offences against socialism begin. The first backbench rebellion of the New Labour government (on cutting benefits for single parents) came just six months after Tony Blair’s landslide victory in 1997. And MPs were generally less rebellious in those days. The Labour leadership is mindful of the need for some mechanism to win consent for hard choices. Sue Gray, Starmer’s chief of staff, has floated the idea of using citizens’ assemblies – members of the public sitting, jury-style, to weigh competing evidence – as a way to navigate policy dilemmas. The method is credited with steering Ireland through referendums on abortion and same-sex marriage that were much less fractious and embittering than the UK’s Brexit plebiscite. Critics say the assemblies end up captured by government or ignored when they return unwelcome answers. MPs, jealous of their own legislative function, mutter that parliament is already the supreme citizens’ assembly. Pledging to delegate decision-making away from the centre is the sort of pious thing parties do in opposition. Once in government, they freak out at the loss of control and start manically hoarding power instead. Proportional representation would be a more radical way to give accurate expression to diversity of political opinion. But Labour’s current polling trajectory snuffs that idea. Any party that wins power through first past the post loses any incentive to reform the system. There are sensible objections to PR based on preserving intimacy between MPs and their constituents. Less persuasive is the claim that politics would be paralysed by a succession of unstable coalitions. The evidence of the past decade suggests otherwise. Single-party Tory rule has been a lot more chaotic and fractious than the formal coalition that preceded it. You can have multiparty rule with explicit deals to share power or, as we have now, submerged coalition politics with warring factions vying for supremacy within one party. The latter model is not obviously more democratic. There is also no guarantee of collaborative spirit flourishing in a more proportionately elected Commons. The culture of tribal animosity runs deep. The rewards for acknowledging that someone on the other side has a point are few. That spirit is even less welcome in the digital arena where debate is dominated by the highly strung, hyperpartisan tendency. Politics in anticipation of the next election is marked by a weird combination of stasis and crisis; turbulent stagnation; fragmentation channelled through a broken electoral system so it looks like stability, defined as temporary respite from chaos. It is hardly surprising if most people tune out. Nor should it come as a surprise if, when they tune in again, there is only one message that cuts through – one line that can be safely deployed in any pub in any constituency. It is the imperative that keeps a lid on all the volatility simmering below the surface of British politics: first, get the Tories out. Rafael Behr is a Guardian columnist