Ian Buruma, in NRC Handelsblad's English edition (http://www.nrc.nl/international/opinion/article2339000.ece):
The usual percentage of people voting for extreme-right parties in Western European countries - whether out of protest or out of conviction - is around 15 percent. Twenty percent is considered high. In the Netherlands opinion polls suggest that 40 percent of Dutch people agrees with the ideas of Geert Wilders. Agreeing with him doesn't necessarily mean that they will vote for him, but it is a serious phenomenon.
It seems too simple to say that Wilders' popularity is based solely on the behaviour of Moroccan youth, as some have suggested. This doesn't explain, for instance, why radical populists get high scores in other European countries. It may be that the British, Swiss, Danes and Austrians have their own versions of the loitering Moroccan youth in the Netherlands, but it is striking how many people who say they are afraid of non-Western immigrants seem to live in villages that hardly have any.
Resentment is usually the source of populism; resentment against the political and cultural elites who are deemed responsible for the decay, the decadence, the flood of alien elements, and the betrayal of the common man. This has happened before. I think the current widespread dissatisfaction has to do wit the consequences of globalisation, the ill-understood - but not imaginary - power of multinationals and political institutions, and the apparent powerlessness of national governments that are perceived as pursuing their own interests rather than those of the voters, who in turn feel like they've been left to fend for themselves in a time of rapid changes. These transformations are felt much more in poor areas than in places where the elite live.
Politicians like Wilders thrive on this general feeling of discomfort. They do this by linking problems like crime, Muslim extremism or the undemocratic nature of multinational corporations to the ever simmering resentment against the 'cultural elites', the 'left-wing church', or whatever they want to call the perceived wrongdoers.