PARIS—Deep divisions in France over antisemitism and the Gaza war have left Marine Le Pen’s opponents at odds on whether to band together to stop her party from winning a majority of seats in the final round of parliamentary elections. The discord was already apparent the morning after Le Pen’s first-round victory, with leaders of President Emmanuel Macron’s party accusing some far-left candidates of antisemitism and refusing to endorse them. Persuading third-place candidates to step aside and provide an endorsement is a crucial step this week if Le Pen’s opponents want to impede her party from winning control of the...