The Sweden Democrats look at first sight like any of the other anti-immigrant and populist parties that have spread across Europe in the last decade. They are said to be Steve Bannon’s favourite party with their programme of xenophobia and cultural nationalism – the 1950s, but with mobile phones, as one of their founders once told a journalist.

They have risen from the extreme fringes to a position where they have completely jammed up the traditional power blocs of Swedish politics, which since 1976 have been based around the competition between the Social Democrats and a grouping of three or four non-socialist parties. None of the mainstream parties will co-operate with them, but neither bloc can form a majority government without them.

Yet on closer inspection, they are a very Swedish phenomenon. In Denmark, Norway, and Finland, parties that share their ferociously anti-immigrant message have long been admitted to government and form part of the normal political process. Swedish mainstream parties, though they have adopted many of the Sweden Democrats’ policies since the great U-turn on immigration of 2015, have simply refused to acknowledge their existence.

Political scientist Olof Petersson says one of the distinctive features of Swedish political culture is that it is both more authoritarian and more democratic than the European norm: in the Swedish model, authority consults widely, but is obeyed when it reaches a decision. There is always an elite consensus at the apex of Swedish society, and at any given time a very narrow range of acceptable views, but these can suddenly change, as did attitudes to the old welfare state after an economic crisis in the 1990s, or attitudes to refugees in 2015. The decision in 1994 to join the EU followed another sudden change of mind at the top of the Social Democratic party, communicated quickly and effectively to an initially unwilling membership.

The drawback of all authoritarian systems is that the people at the top will hear only what they want to, and gradually lose touch with reality. What prevented this in Sweden for most of the 20th century was mass membership of political parties on a scale hard to imagine in Britain. In 1983, nearly a quarter of the population was enrolled in a political party. More than 1 million people out of a population of 8 million were members of the Social Democrats (I was myself, as the union representative in the small factory where I worked).

The parties functioned almost as social media do today, says Petersson. Young people joined to meet others, especially in the country. And because they were so embedded in everyday life, party officials had a very good idea of what ordinary, unpolitical people were talking about. Since then, membership has collapsed to levels scarcely above those of British counterparts. About half the membership of the Social Democrats vanished with the abolition of collective membership through the unions. By 2010 the party had only 109,000 members. There was no longer any mechanism by which the leadership on Stockholm could be told things they did not want to hear.

That was the void in which the Sweden Democrats flourished. Originally a neo-Nazi sect, which was taken over and cleaned up by four student politicians, it was the only party that spoke out against the consensus on immigration. The mainstream parties tried for years to ignore them entirely.

Just before they entered parliament for the first time, in 2010, I asked the minister for integration what her plans were for when they got in (as the polls showed they would) and she replied that it was unthinkable they should. No thought was duly given. So now the party has grown to be the third – and will quite possibly soon be the second – largest in the country.

But it is still a captive of the authoritarianism from which it benefited. Unlike true fascist movements, it has no coherent plan to reshape the state, and no views on capitalism. It just believes the wrong people are running the right system. In that sense it is already a part of Swedish political life, whatever the result of the election.

Andrew Brown is the author of Fishing in Utopia (Granta), a memoir of his life in Sweden that won the Orwell Prize