PARIS—What’s striking about the French presidential election is the extent to which the two front-runners share a basic analysis of the choice facing the country. Marine Le Pen, the leader of the right-wing National Front, and Emmanuel Macron, the 39-year-old former economy minister who quit François Hollande’s government to stand as an independent, are poles apart politically. But both agree that the defining issue is France’s membership of the eurozone.

Both point to the widening divergence between Germany and France’s...