It is not a sudden panic about the threat of environmental catastrophe that has caused the Green party to swell in membership and hit up to 11% in the polls. It has everything to do with the failure of Labour to offer an inspiring alternative to five years of Tory austerity. According to IPSOS Mori, more than a quarter of Britons believe that Labour plans to “cut spending too much”. It just takes a few of those to defect and Labour is in trouble. The Greens offer a living wage, public ownership of rail, higher taxes for the rich, and other policies based on social justice that appeal to large swaths of the electorate.

There is one obvious counterattack Labour can make: vote Green, get a Tory government. Because of the absurdities of first-past-the-post, the Greens are unlikely to win any seat other than Brighton Pavilion, already held by the courageous and principled Caroline Lucas. According to Ladbrokes, the Green surge could deliver seats such as Brighton Kemptown, Hove, Stroud, Norwich North and Bristol North West to the Tories. Losers could include Labour leftwingers such as Brighton Kemptown’s Nancy Platt, Norwich South’s Clive Lewis, and Lancaster’s Cat Smith.

That will mean five more years of the bedroom tax, the dismantling of the NHS and the stripping away of the welfare state. If Labour loses, Ed Miliband’s successor will almost certainly drag the party to the right, and many Green voters will suffer “buyer’s remorse”: the exhilaration at voting for principles will be eclipsed by despondency at being saddled with another Tory government, deterring them from voting Green again. Few of those who claim there is no meaningful difference between a Labour and Tory government are being hammered by the bedroom tax.

Such a strategy will not work. The past year has surely demonstrated that the “politics of fear” is counterproductive. Instead, Labour must respond with hope. A critic would argue that it is rather too late in the day for any new policies to gain traction. But there are three clear commitments Labour could offer to win over Green defectors. First, renationalise the railways. It would cut through like few other policies, and probably prompt some voters to break out in spontaneous applause. Polling demonstrates a publicly owned railway has near-universal appeal, winning over well-heeled Tory commuters and Ukip voters alike. But it also has a totemic quality about it: a clear demonstration that Labour has taken a decisive stance against the untrammelled market in the era of market failure.

Secondly, a living wage phased in by 2020. Labour’s current offer – of £8 an hour by 2020 – is, to be blunt, pathetic and derisory. It will continue to consign millions to working poverty, with the state continuing to subsidise low pay at vast expense. A living wage would inspire Green voters who want a radical response to years of collapsing living standards.

Finally, a commitment to offer a referendum on proportional representation. A confession: I’ve been on a journey on this myself. Like most Britons, I voted no in the 2011 Alternative Vote referendum, partly because of a belief that permanent coalitions would enshrine mushy centrism and give politicians a perpetual excuse to discard election promises. But given that first-past-the-post no longer seems capable of delivering stable majority governments and the two-party hegemony is apparently over, coalitions are here to stay anyway.

A commitment to offer PR in a referendum would be telling Green voters, “You are rightly frustrated that you cannot vote according to your principles, but vote for us and we will allow you to do so in future. If the Tories win, then forget it.” Sadiq Khan, a key Miliband ally, has been tasked with taking on the Greens, and is a supporter of PR. His colleagues would do well to listen to him.

Rail renationalisation, a living wage, a referendum on PR: it is an offer that many Green voters would find irresistible. There are only three and a half months to go. British politics is in meltdown, and timidity simply will not do. Over to you, Labour.