PARIS — When Carlos Ghosn was arrested on his private jet in Japan last month on charges of violating the nation's financial reporting laws, Nissan promptly removed him as its chairman. At the French automaker Renault, the management stuck by Mr. Ghosn, its chief executive and chairman. But an intensifying storm is putting pressure on Renault's defensive posture. Mr. Ghosn was detained just as Renault was suffering a slump in sales in Europe and confronting a worsening power struggle with Nissan, its longtime partner. There is even a political dimension. In the Yellow Vest protests that have galvanized France, Mr. Ghosn has become a symbol of excess wealth and runaway corporate power. Renault's position is becoming increasingly untenable the longer Mr. Ghosn sits in jail.