When the left tries to combat the right by moving to the right.
Austerity, cutting government spending and hoping that the private sector will take up the slack.
Giving an economy with high long term unemployment especially amongst the young and low if any growth.
During this time what happens on the right, they move to the right and find social scapegoats, immigrants, gays, and poor people even though the root cause is the continuation of their economic policies.
Very low voter turnout for France of around 61 to 62% since many on the left stayed home due to apathy or disgust with the ruling classes.
The far right as is often the case improved their position even without improving their general appeal. Marine LePen has made the Front Nationale slightly less vomit inducing than it was under her father but it is still as nasty underneath as it always was.
This is the perfect case of a center left wing government trying to follow on from a center right wing government by applying the same economics. The young have been pretty much abandoned in this climate with an unemployment rate of twice the national average standing at around 26%. We have had a jobless mild recovery.
The result of all this is that the President François Hollande is about as popular as head lice and the left completely demotivated. So when an election came around the expected result occurred because so many on the left said fuck that, and stayed at home.
So the Front Nationale made some gains but the real winners in this were the center right parties and if this continues into the Presidential elections in 2017 we will just have the same economic policies that made François Hollande so unpopular again. The crux of the matter that is exactly what the right will promise will work if tried hard enough [it never does] whereas the "left" promised something different but did the same.
The conclusion of this for the "left" is don't try and be a right wing party economically [especially since you know those economic policies have been poor at best] yet still expect a different result.