En Marche benefitted from the peculiarity of France's electoral system, which combines proportional representation in a 1st round with a "sudden death" 2nd round for the powerful presidency with only the two best-polling parties taking part. When it became clear that the far-right would be the largest single party, people ditched traditional party allegiances to back a candidate they found tolerable.
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(Apologies for posting my half-mangled recollections without checking and thanks to the commenters who fixed my incorrect claims).
France's electoral system is not well-suited to its current political landscape.
> which combines proportional representation in a 1st round
No it doesn't.
The french parliament is elected in 577 single member constituencies by a 2 round system: in the 1st round, voters vote for any candidate.
If no candidate wins >50% of all the votes in that constituency, a 2nd round is held, where candidates that came 1st or 2nd or got >12.5% of the vote, in the 1st round, can run.
Thus the French 2 round system is similar to Instant Runoff Voting, except that it isn't instant.
As an example of how the French system isn't proportional, En Marche got 32% of 1st-round votes but 61% of seats.
> for the powerful presidency
Presidential elections, like any other election that elects 1 candidate cannot in principle be proportional, because each party must either win 100% or 0% of the candidates elected.
> When it became clear that the far-right would be the largest single party
No, the National Front only got 13.2% of 1st round votes (and only ended up winning 8 seats, 1.4% of the total). They were nowhere near being the biggest party.
That's not entirely correct. Macron got 24.01% in the first round, 66.1% in the second. Le Pen, the far right candidate, got 21.3% in the first and 33.9% in the second. The runner-up in the first round was Fillon, a coservative, with 20.01% in round one. There was the fear Le Pen would win the second round, sure. One thing about the French system is, that usually the second round is not won by a radical candidate as those usually fail to get enough votes from the other parties and candidates not qualifying for round 2. We'll see how that plays out next year so. Not sure how the French system wouldn't fit the current political landscape.
(Apologies for posting my half-mangled recollections without checking and thanks to the commenters who fixed my incorrect claims).
France's electoral system is not well-suited to its current political landscape.