—The improbable rise of Emmanuel Macron as the youngest president in modern French history – held up as a rebuke to transatlantic populism and generating expectations from Boston to Berlin – started with the big hope of women like Christelle Dernon. Ms. Dernon was one of thousands of French people who signed up for the political movement “En Marche” that Mr. Macron started just a year ago, knocking on doors across the country and helping create and curate the message of optimism that ultimat...