Well, I am not that surprised that he has so many supporters. Due to 9/11 and even more so by the election campaign of the (murdered) Pim Fortuyn, the murder on filmmaker Theo van Gogh (by a muslim extremist), the death threats that Ayaan Hirsi Ali, Geert Wilders, Eshan Jami and others received, the occasional terrorist cell that is discovered, other muslim extremists like Samir A and the Hofstad group who had plans to blow up the AIVD agency and other places etc etc etc, there is a LOT of press attention to anything related to the islam and sadly it usually is negative attention. That many islamic youngsters are causing trouble in neighbourhoods of Amsterdam, Rotterdam, The Hague, Utrecht etc, in swimming pools, synagoges, to gay people, women an more makes it rather easy for a populist like Wilders to take advantage of the present fear among the Dutch. He usually has strong and understandable (but IMO totally rasict, counter-productive and foolish) remarks like ´the Koran is a fascist book like Mein Kampf and has to be forbidden´. At the moment he is making a movie about the islam, much like Theo van Gogh did and which was the reason why he got murdered.
Add to this mood etc. the recent change of the press towards more sansationalism, hyped items and more of the sort (a trend since they started with commercial television in the early nineties) and it will result in loads and loads of hyped items in the press about Islam, Dutch identity and more of the sort. The royal family has been a ´victim´ of the new press fodness of hypes every time something sensitive happened, as we saw during the news around Mabel, Margarita, Maxima on the Dutch Identity and the recent christmas speech (among others). I summed up the negative ones but it also works the other way around, the RF has a LOT of press attention the last years, also by the serious press. IMO the 40th birthday of the CP (the actual birthday, not the party in September) was hyped by the press too, the newspapers and television station stumbled over each other to say how he is ready for the throne, what a good vision it was by the late Prince Claus to advise WA to start about water while we all laughed at it then, etc.
Anyway, this Wilders has a new competitor since former immigration minister Rita Verdonk split up from the liberal party and started her own party ´Proud of The Netherlands´ (TON), a most charactaristic name for a political party to start in this decade. She is very popular among a large part on the right side of the political spectrum but much less extreme than Wilders, she openly said more than once that she found Wilders style and ideas far to extreme and focussed on the negative. (Note that I am not a fan of Verdonk either but at least she has some decency and common sense).
Anyway, since Verdonks entry Wilders is getting less and less support in the polls and a shocking result of 25 seats in the polls did deminish to 15 at best in the ones now. I suppose that critisism on the Queen and the RF has no advantge for a political party, traditionally the Dutch do not like politicians to be too severe about the royal family, so labour, the christian democrats and the liberals usually support the monarch in these things. The new star on the block, the former Mao-ist party of the Social Democrats only gained the landslide victory AFTER they changed their programme on the monarchy (they no longer want The Netehrlands to turn into a republic). In general I think that this little-poor-me thing of Wilders vs. the Queen will backfire, especially because his voters usually are conservative voters who in general are the most supportive ones of the monarchy.
Another liability will be the result of the movie he is making. If the reaction will result in a crisis like in Denmark or even terrorist attacks (De Telegraaf interviewed a few imams who said ´what you need is a good terrorist attack´, in relation to this film).
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Anyway, this will eb away, it usually does. Going to the next hype and in a few years we will get another politician from a fringe party that wants to limit the Queens role. Again polls, discussion etc but nothing will change. I have seen this happen in the past too, though usually done by D66 (social-liberals).