Whatever became of the Green Party? There was a time when the “Green surge” promised to be one of the big surprises of 2015. Boosted by the controversy around who would take part in televised debates, the Greens gained 2,000 new members in a single day in January, taking their membership close to 45,000, more than the Liberal Democrats or Ukip. They now claim a membership of 60,000, four times what it was less than a year ago.

In the same month, one of Lord Ashcroft’s opinion polls put their support at 11 per cent, pushing the Liberal Democrats into fifth place. Now, the polls put the Greens on 5 per cent. Speaking to The Independent, their leader, Natalie Bennett, appears to attribute that fall in support to Britain’s two-party system. “People have been trained to vote tactically over generations,” she says.

That may be the story of past elections, but not this one. Support for the smaller parties has never run higher. The SNP is surging ahead in Scotland. Ukip’s support is holding up, despite predictions that it would melt in the heat of an election. Even the Liberal Democrats have improved their meagre standing since January.

Meanwhile, half of those thinking of voting Green have changed their minds in three months, despite the exceptional exposure of two live television debates. The reason is not difficult to deduce. In a crowded market, a political party has to have one big idea that it can push to the centre of the debate and by which it is immediately defined. The SNP has Scottish home rule. Ukip has immigration. In the past, the Lib Dems had opposition to the Iraq war and to tuition fees – though not any more. The Green Party has a big idea – concern for the environment – but failed to get it on to the mainstream political agenda. It is almost as if it did not try.

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Ms Bennett, as the party leader, has to carry the can, even if she did not create the democratic structure which gives an army of well-meaning party activists their individual inputs into party policy. The Green manifesto is the longest on offer in this election, almost twice the length of the Labour Party’s, and replete with mildly eccentric pledges such as banning rabbit cages.

When the “Green surge” was just beginning, Ms Bennett endured an excruciating interview with Andrew Neil, in which she floundered, unable to explain how her party proposed to fund the £72-a-week universal basic income it proposed to pay to every man and woman in the country – with a smaller sum for children – at an estimated annual cost of £280bn. The Greens have since admitted that it “would not be practical or right to carry out that change within a single parliament”. They could have saved Ms Bennett a lot of grief if they had acknowledged that sooner.

Though that interview was bad, it was not Ms Bennett’s worst. That was the notorious “brain fade” she suffered when asked a gentle question on LBC about how the Greens would pay for their promise of 500,000 new social rented homes. She claims that the episode is now “ancient history”, implying that only journalists remember it.

Even if that is true, her “brain fade” was just part of an overall failure. The Greens lumbered themselves with too much policy, chose a leader who did not have the sharpness of wit to handle the details under pressure, and allowed their main message to be lost. This election was their biggest opportunity. They have squandered it.

The Independent has got together with May2015.com to produce a poll of polls that produces the most up-to-date data in as close to real time as is possible.

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