Open this photo in gallery: Founder of left-wing party La France Insoumise (LFI) Jean-Luc Melenchon walks during a demonstration to support Palestinians and to demand for a ceasefire in Paris on June 8.SAMEER AL-DOUMY/Getty Images When French President Emmanuel Macron called a snap legislative election this month, he was counting on bitter divisions among political parties on the left to enable his centrist coalition to hold onto a plurality of seats in the National Assembly. Like most things these days, however, this ultra-brief campaign has not been going Mr. Macron’s way. The four main parties on the left have managed...