MARSEILLE, France—Sitting in his spacious office overlooking the city’s Vieux Port, where ships have come and gone for more than 2,600 years, Marseille Mayor Benoît Payan acknowledges something that may have once seemed unthinkable: The far-right National Rally (RN) stands a real shot at capturing his seat in March. This longtime hub for immigration is reputed for its autonomous spirit—as the adage goes, locals identify more as Marseillais than French. But as the incumbent left-wing mayor