I'm not sure En Marche are actually going to win big...latest projections have them at c22 MEPs. Largest party sure, but Le Pen's group are projected to get c18, Republicans c14 so it's not *that* big a win compared to who comes 2nd/3rd. Plus if current trends continue then Macron's popularity will potentially fall even further by the time the vote actually happens...next French Presidential election is 2022, Macron will still be pushing through unpopular things in 2019 as it's the middle of the electoral cycle.