National Front leader Jean-Marie Le Pen has called for all of his supporters to abstain from the upcoming presidential runoff. Mr. Le Pen is considered by many to be a fascist and a racist bigot that is discontented with France's immigration policies. The opposite of Le Pen's ideology can be found in that of Madame Royal, the Socialist candidate that stands to gain from Le Pen's call.
So, why would Le Pen deliberately move in such a way as to strengthen one that is such a polar opposite?
Maybe, Le Pen is waging his bets that a Royal presidency will result in a further decline into what he feels is already an abyss, as far as French policy goes. If Royal makes more of a mess of the republic than what he feels has already been made, he or his movement will be strengthened more in the future. The one thing that fascism depends on is, reactionaries need an enemy (and that usually means communists and socialists) to react to. Mussolini was able to gain power by using the inefficiencies (brought on by the futile attempts to implement their failed ideology) against them.
If France elects Royal, that is what Le Pen is hoping will happen here.
As a footnote to all of this, Anne Applebaum of WaPo has an interesting analysis of who Sarkozy is appealing to.