The problem with the Green party is that it is too nice. They don’t hate, and if left-wing politics in this country needs anything it is a dose of rigorous hate.

The Greens succeed despite this impediment. Their membership has doubled since January. They continue to rise in the polls, to 7% or 8%. And there has been plenty of opportunity for this success to be proved a fluke. After Caroline Lucas’s election victory in 2010 the party could have lost momentum. Instead, they won Brighton council. After the toxic fall-out from implementing austerity, however reluctantly, it would have been natural for them to be fatally tainted. Instead, they continue to grow in a terrain which is decidedly unfavourable to the left.

What is remarkable about the Greens is that they manage to do this while maintaining some quite unfashionable stances. They won in Brighton despite distancing themselves from local bigotry against Travellers. They surge nationally now despite taking a relatively progressive stance on immigration. Somehow it never occurred to them that they had to start hyperventilating at the sight of a white van.

What is even more remarkable is that the Greens are not, like Ukip, an “antipolitical” party. Nor are they total outsiders: they have exercised power at various local levels; they have worked with Labour in London; they are against austerity, but its elected representatives have shown that they will implement it rather than engage in a neo-Poplarist act of defiance – they are not pretending they are going to overthrow the political class.

This could be both their strength and their weakness. As an electoral vehicle, they seem sane, realistic and progressive. They also don’t look like they’re about to collapse into a puddle of blood at any moment, like so many other organisations of the left. Yet, it is doubtful whether, on that basis alone, they can expand beyond a still relatively middle-class ghetto.

The fine, humane social democratic bromides that the Green website extols – “the common good”, “fairness”, making politics work better – are congruent with their general good-humoured parliamentary politics. They play the ball, not the person, as it were. This might work in another time, another place.

‘Ukip succeeds because it canalises a great deal of resentment and seething anger.’ Photograph: Gareth Fuller/PA

The problem is that Britain today is an angry country. It is filled with seething hatred, and justifiably so. There is no end of social misery and resentment caused, not just by the crash and the austerian aftermath, but by the insulting ease with which the financial masters got away with it and continue to accumulate gargantuan, metastasising bonuses.

Those who talk of a “Ukip of the left” should learn from what that ruthless party of Poujadism does so well. Ukip succeeds because it canalises a great deal of this resentment and seething anger – the desire to cut the politicians and bankers off at the knees – into a political vehicle that speaks to the anger of abandoned Tory voters.

What about abandoned Labour voters? How does one speak to their anger? It is excellent, but not enough, for the Greens to say they won’t scapegoat immigrants and other folk-devils. If immigrants aren’t to blame, then we need to know who is to blame.

Left-populist movements in Europe that succeed tend to know who the enemy is, and name it. For Syriza it is the troika; for Podemos, it is la casta or the caste, their term for the parliamentary elites, businessmen, media elites and bankers who dominate Spanish society. These are simple, common sense terms designating the object of furious popular hatred. They are also concrete. Anyone could name prominent representatives of the troika or la casta, people whose mere appearance on a television screen is enough to set teeth grinding.

The lack of such a popular idiom in England is hardly the fault of the Greens, but they have this problem more than anyone else. Even if they secretly yearned to see the bankers roughly expropriated, the rich sadistically taxed and the political class donated to a landfill, to say so would grate against their consensual ears.

Not being prepared to get their hands dirty, too committed to the niceties of parliamentary politics, they risk looking as aloof and out of touch as the establishment they aim to upset.

If they genuinely want to get ahead, they need to discover their dark side.