Had Theresa May won Thursday’s election convincingly, the Green party would now be staring into the abyss. That the massively talented Caroline Lucas was re-elected in Brighton Pavilion with an increased majority would have cheered the party’s supporters, but not enough.

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The most obvious reason for this is that the party failed to win, or even to be the runner-up, anywhere else. Molly Scott Cato in Bristol West, where the party came second two years ago, was the great hope, and her third place behind a colossal Labour vote and a much smaller Tory one was disappointing. Losses for former party leader Natalie Bennett in Sheffield Central and Vix Lowthion on the Isle of Wight were also heavy, with the Greens in Sheffield slipping back, as in Bristol, from second place to third.

But more ominous than the fact that the Greens appear stuck on one English seat was the thinly veiled, and so far little remarked upon, threat in the Conservative manifesto to try to kill them off. Brexit was already a body blow for the Europhile party with three MEPs. Without Brussels as a staging post, it is hard to see how Lucas would ever have made it to Westminster. Brexit means no more MEPs, and that route to office blocked.

With its promise to change the voting system in mayoral, crime commissioner and London Assembly elections from a proportional to a first-past-the-post system, May’s manifesto set out to raise the barriers to Greens and other small parties beyond a level they could reach. If these changes go ahead,as it is still possible that they will – although the Tories’ weakened position makes it less likely – the Greens lose five elected members: three in Europe and two in London. After decades building itself up, there is a real question as to how the party could recover.

To a Green party member such as me, albeit one who voted Labour this time in the belief that my MP Karen Buck was at risk, it is a relief to see May thwarted. But would the Greens and other small parties – the Liberal Democrats, Ukip and Women’s Equality party – fare any better under Labour?

Its manifesto promised a constitutional convention and an elected House of Lords but said nothing about proportional representation. The danger is that with a majority now within reach, thanks to the Tories’ spectacularly bad record, Corbyn’s Labour will be tempted – just as New Labour was – by the status quo.

Some will say: who cares? After all, we can hardly redo parliament at the same time as Brexit. The problem is that small parties are not leaving the UK. In Northern Ireland and Scotland they run things, and now we have a UK government set to be propped up by the DUP. In this context, to speak of the return of two-party politics is nonsensical. Yes, Labour and the Tories between them won 80% of the votes last week, a level not seen since the 1980s. But parties with a tiny fraction of these votes wield great power too.

Fewer than a million voters managed to elect 35 SNP MPs, while 2.4 million Lib Dems only got 12

My point is not simply that we need electoral reform, although we do. It is that the system we have evolved, combining first-past-the-post with devolution, gives disproportionate weight to nationalist and Northern Irish unionist parties while excluding environmentalists, feminists, liberals and Ukip too. To give one example, fewer than a million voters managed to elect 35 SNP MPs, while 2.4 million Lib Dem voters only got 12.

It seems foolish to deny that an overhaul of the electoral system, at this point, seems highly unlikely. But anybody who thinks our cranky system has proved its worth by so effectively denying Ukip voters representation should reflect on the party’s astonishing success. First-past-the-post, I would argue, in fact enabled Brexit, by amplifying the internal struggles of the Conservative party to the point where they took over the country (in a proportional system, a party with 37% of the vote would never have been in charge on its own).

May lost last week, and most Greens will be jubilant despite the party’s lacklustre performance – not least because many voted tactically to support a “progressive alliance”. But this was a narrow escape. Had May won, she would have set about reversing the baby steps towards PR of recent years, and introduced a rule requiring voters to show ID – a measure expected by experts to reduce turnout, particularly among younger voters, and strongly associated in the US with voter suppression. One can only hope Labour and others will strenuously resist any attempt to deliver on these antidemocratic promises.