All political parties face a trade-off under a first-past-the-post electoral system. Governing depends on attracting a broad coalition of voters, inevitably involving compromises that leave a party’s base disgruntled. So it is perhaps unsurprising that as we move closer to a general election, the discontent from the anti-Labour left who claim there is little to distinguish Keir Starmer from Rishi Sunak in the battle for the premiership is only getting noisier. They have upped the ante in recent weeks. An umbrella campaign, “We Deserve Better”, that aims to unseat the shadow culture secretary, Thangam Debbonaire, and the shadow health secretary, Wes Streeting, among others, is channelling donations to Green and independent candidates, including none other than the former Labour leader Jeremy Corbyn. The argument is threefold: there’s no meaningful difference between the Conservatives and Labour; Starmer supposedly can’t be trusted because he has dropped pledges he made in the 2020 leadership election to shift his party towards the centre; finally, the “Tories are toast” and Labour can’t lose, so disgruntled left voters can safely vote for other parties, such as the Greens. With Labour so far ahead in the polls, the urge to debunk these sentiments may seem like an expression of paranoia. But all three aspects of this narrative are comprehensively wrong, including the reassurance that it is safe for anyone who would prefer a Labour government to vote for another party in Labour-Tory contests. Amid a very positive set of results for Labour in the local and mayoral elections earlier this month, the party experienced losses in some of its safer seats as a result of disaffection, including over its position on Gaza. So while it is true that there are few existing Labour seats at risk, those that are include Debbonaire’s Bristol seat, where the Greens saw a breakthrough, and potentially Shabana Mahmood’s in Birmingham Ladywood, where the independent candidate for West Midlands mayor is running, and Jess Phillips’s seat of Birmingham Yardley if an independent were to stand against her. It would be a huge loss if any of these able and widely respected feminists were unseated. But what this underplays is the number of Labour-Tory marginals where a relatively small vote for other left candidates could cost Labour a win. James Kanagasooriam, of the polling company Focaldata, has written about the “sandcastle” nature of Labour’s likely majority; his forecast is that there will be many more marginal seats in the 2024 parliament compared with 2019. If more than predicted numbers of those who voted Green in the locals decide they can afford to do so in the general election because Labour is so far ahead in national polls, that will boost the Conservatives. Next up is the idea that Starmer’s dropping of some of his leadership pledges makes him dangerously untrustworthy. But this is the product of a system in which the tiny unrepresentative slice of the electorate that is a party membership pick their leader before voters choose their prime minister. Anyone hoping to be PM would have to shift position between a leadership selection and a general election: a Labour leader’s most important job is to connect with potential voters, not to coddle members with the comfort blanket of a policy platform such as the “free broadband for all” 2019 pledge that was roundly rejected. Liz Truss provides a cautionary tale of what happens when a party leader seeks to impose a membership-endorsed platform on the country without a general election. For Starmer to have stuck to his 2020 leadership election pledges, instead of spending the past four years understanding voters, would have been fundamentally anti-democratic. The most egregious aspect of the anti-Labour left argument is there isn’t much to choose between Starmer and Sunak. Yes, Labour’s “Ming vase” election strategy has seen it take a much more cautious fiscal approach than many of us would like: it has effectively adopted the Tory macroeconomic worldview and with it a set of spending constraints that no one sensible thinks either party could stick to in the wake of the election. That is frustrating for anyone hoping this election campaign may illuminate some of the tough trade-offs facing Britain; but it would have been incredibly risky for one side to go it alone on this. The alternative is Labour walking into the trap and handing the Conservatives a “Labour tax bombshell” election campaign. From a commitment to scrap the Rwanda plan to making clear that in an ideal world Labour would discard the two-child benefit cap, there are plenty of reasons that it is preposterous to think that a Starmer government would make the same trade-offs as successive Conservative governments that have financed billions of pounds worth of tax cuts for more affluent families by cutting tax credits and benefits for low-income parents. The six pledges Starmer launched two weeks ago may be incremental, but Labour needs voters to believe they are deliverable, and they are indicative of a very different set of priorities than those that animate Sunak. Finally, there has as of yet been less scrutiny of the independent and Green candidates trying to appeal to Labour-sceptical voters than there has of Reform on the other side of the spectrum. It is not just Labour that has changed: the Green party has mopped up enough of the cranks expelled by Labour that it has had to acknowledge its growing problem with antisemitism. Akhmed Yakoob, the independent who came third in the West Midlands mayoral election and who is now running against Mahmood, promoted a video that was manipulated to falsely suggest a teacher campaigning for Labour used racist language, which prompted a viral hate campaign against her. (Yakoob says he did not intend to cause distress and that the video was hard to hear.) Starmer is not without weaknesses, as shown by the days he took to clarify an interview last October in which he gave the impression he thought Israel had the right to withhold power and food from Gaza. But there is no doubt whatsoever he would make a vastly more compassionate and competent prime minister than Sunak. To encourage people to put that outcome at risk by casting a protest vote against a Labour government that does not yet exist is perhaps the ultimate form of luxury belief campaigning. Sonia Sodha is an Observer columnist