Over the last three decades, far-right parties in Europe have tripled their vote share, from about 5% in the early 1990s to more than 15% today. About one in six Europeans now vote for parties such as Marine Le Pen’s National Rally (previously National Front) in France, Matteo Salvini’s League in Italy, or Viktor Orbán’s Fidesz in Hungary. This is one of the most striking outcomes of a new edition of the PopuList, a research collaboration supported by the Guardian, which launches on Tuesday.

Political scientists have demonstrated that the single most important reason why people vote for far-right parties is their attitude towards immigration. In other words, those who support these parties tend to do so because they agree with them that immigrants are “dangerous others” who form an economic and/or cultural threat to their own native group. This in itself is not remarkable – it just shows that many supporters of far-right parties are rational voters in the sense that they opt for outlets that express ideas they agree with and deem important.

What is remarkable, however, is that people’s attitudes towards immigration have not changed much over the years. On average, people have become neither more positive nor more negative about the influx of immigrants. This confronts us with an interesting puzzle: how is it possible that the electoral successes of far-right parties have increased so much, while at the same time the main determinant of far-right support – anti-immigration sentiment – has remained relatively stable? Let me mention three developments that can help us understand what is going on.

First, far-right parties have learned how to better mobilise voters. Until the turn of the millennium, most far-right parties were generally conceived of as beyond the pale. Voters associated them with fascism and violent anti-democratic skinheads. This has changed as many far-right parties have succeeded in moderating their images. A good example is Le Pen’s “detoxification” strategy. She cut connections to extremists, changed her party’s tarnished name and distanced herself from her father, the more radical previous leader and founder of the party. It is important to emphasise, however, that although various far-right leaders have succeeded in moderating their party’s reputation, when it comes to their actual programmes most of them have remained as radical as ever.

Facebook Twitter Pinterest A far-right rally in Rome, Italy. Media coverage has made immigration one of the most widely debated issues in Europe. Photograph: Stefano Montesi - Corbis/Corbis via Getty Images

Mainstream parties have also played an important role in this process of reputation moderation. They have legitimised the ideas of far-right parties by incorporating watered-down versions in their own political programmes. Many academic studies have shown that when it comes to their positions on immigration and integration, mainstream parties have moved towards the far right. The Dutch prime minister, Mark Rutte, for instance, wrote a letter to all citizens in the Netherlands, in which he argued they had to actively defend the country’s values against people who refused to integrate or who acted antisocially. “Behave normally or leave,” was one of his letter’s core messages. Similar strategies have been employed by mainstream parties in Austria, Denmark and France.

Second, it is not only political parties that have adapted their behaviour – voters have also changed. Not so long ago the average voter was loyal to their political party; a typical social democrat remained a social democrat forever, and did not even consider voting for a conservative or liberal party. Yet processes of individualisation and emancipation have made people much more whimsical when it comes to their electoral behaviour. Slowly but steadily, loyal voters became floating voters, and many of them started to switch between parties – in particular in multi-party democracies. These voters often have a “choice set” of several parties to which they feel attracted, and base their eventual choice on things such as election promises, the performance of party leaders, or parties’ past behaviour in parliament. Voters have started to actually vote, and have thereby become available for mobilisation by far-right parties.

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Third, the issue of immigration has become increasingly salient. As a consequence of the large societal impact of immigration-related events such as, for instance, the refugee crisis and the Brexit referendum, journalists have become obsessed with the topic. It has become one of the most widely debated issues in the European media. As a result, citizens are increasingly exposed to immigration-related news and discussions, and this has probably activated their ideas about the topic. In other words, although voters have on average barely changed their ideological positions, the issue of immigration has become more important to them. Most importantly, it is likely that this increased salience has also made their attitudes towards immigration more consequential for their voting behaviour.

To sum up, voters with anti-immigration attitudes have become increasingly likely to vote for far-right parties because these parties have managed to create a more moderate reputation; more voters have become available for far-right mobilisation; and the core issue of these parties – immigration – has become more salient. There is a much better match now between the supply and demand sides of anti-immigration politics.

On the one hand this is good news for democracy, because it gives those who previously felt neglected a political voice. On the other hand it is bad news, because several of the ideas of the far right are incompatible with core democratic values such as pluralism and the protection of minority rights. What this means in practice can be observed in countries including Hungary and Poland, where far-right governments are destroying their (formerly) liberal democracies.

Let us therefore hope that the exponential growth of far-right parties’ vote share will soon come to a halt. A little far right is good for democracy. But too much is disastrous.