Matt Yglesias of Vox points out on Twitter:

You see in Trump vs Le Pen once again that authoritarian nationalist movements only win with the support of the establishment right.

There are two particular reasons why this may be the case.

First, Macron’s party, En Marche!, currently has zero seats in the French parliament. A Macron victory doesn’t have to mean the disempowerment of everyone else. In fact, it gives each party supporting him a seat at the table — assuming En Marche! is unable to manage the impressive feat of winning control of the National Assembly next month.

Second, France’s system encourages temporary polarization, not permanent, structural polarization as in the United States, where one party wins and one is utterly defeated.

The first-past-the-post electoral system has generated two lasting, dominant parties, meaning that individual politicians risk ending their careers if they defect even for one presidential election. Moderate Republicans have lost primary renominations due to lesser offenses.

This forced polarization is unknown in France, where the two-round system allows third parties to thrive without playing spoiler. Leading parties on the left and the right are therefore able to challenge their ideological cousins without handing the final election over to the opposition.

And in the case of an authoritarian nationalist movement getting to the second round, the general lack of polarization meant it was structurally possible for establishment conservatives to reject Le Pen. Her party was a pariah and had not formally seized control of their factions — it just managed to squeak into the second round, where it could be easily crushed at no risk to the general political fortunes of conservatism.