Complex sentence: It will be the second open primary ("primaires citoyennes") held by the parties following the primary in 2011, where François Hollande defeated Martine Aubry to become the Socialist party nominee before going on to defeat incumbent president Nicolas Sarkozy in the 2012 presidential election.
Split into simpler sentences: It will be the second open primary ("primaires citoyennes") held by the parties, after the primary in 2011, where François Hollande defeated Martine Aubry to become the Socialist Party nominee. Hollande went on to defeat incumbent president Nicolas Sarkozy in the 2012 presidential election.