The issue with nationalists is that they seldom stay "national", they usually degress into "us vs them" in which the "us" part becomes increasingly narrow until just a select few stay behind in it.
We had a somewhat likewise pull to the right a decade or 2 ago. Nobody of these fellows got any mandate (by a rather humerous truce of all other parties called "cordon sanitair") and they are still squaring around the same old story that nobody buys anymore.
Same will happen to FN when people start noticing that "the change to politics" is just a new brand of "wait, we promised that? ooo no, can't remember that, please forget that and have some taxes to make me richer".
The US vs them has already started, first it will be us the non Muslims vs Muslims, they've already said quote a bit on France vs Rest of the Europe, then things will evolve to us natives vs immigrants, then us vs Jews (like her father) and so on and so forth.
well it has started in the "election race", but it will eventually spill out into the elections themselfes. sadly.