The French system is a two round system, like Louisiana.
In the first round premier tour)there are many parties and many candidates. If no candidate gets 50.1%, there is a runoff a week later between the two candidates with the highest score.
It is thus impossible for a third party candidate to get into the runoff.
Michele Alliot Marie, the Defense Minister, threw her support behind Sarkozy before the nomination yesterday, so she isn't running.
What happened in the last election was that the two candidates with the highest score in the first round were Jean-Marie le Pen and Chirac.
Since Sego and Sarko have been pulling much higher numbers in the polls than in the previous election, the possibility of a third party candidate beating out either one of them is highly unlikely.
But then again, if anything REALLY big and nasty happens in France between now and the elections, it will boost Le Pen's numbers, and that would hurt Sarkozy.
Royal has hit all the cylinders she can hit. She's declared everything she stands for. Sarkozy's still got some questions surrounding his candidacy - namely, the extent to which Chirac and/or de Villepin will work behind the scenes to damage his candidacy.
Le Pen peaked long ago, unless there is a mass-casualty terror attack in France.
But time does not favor Royal. The French economy has been steadily improving and adding jobs, and the streets have been quiet. The fact that Chirac, whom everyone knows is corrupt as sin, hates Sarkozy actually helps Sarkozy in public opinion: he will not be "More of the same". But of course if the old, dying King decides to lash out at Sarkozy within the power-base it could still cripple him.
Many things are unclear.
With time, Sarkozy will become the natural, obviously leader of the center right, and this harms Chirac's ability to really hurt him internally.
With time, the improving economy makes the Right look smarter and less callous.
With time, the potential for another Islamist explosion increases, and that would hurt Royal.
It will be interesting.