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.

His chances of being released soon from his tiny prison cell in Tokyo, where he has been held since Nov. 19, dimmed considerably on Sunday after a Japanese judge ruled that he could be held at least through Jan. 1 and perhaps longer, to give prosecutors more time to question him. Before that, on Friday, the authorities arrested Mr. Ghosn again based on new allegations of financial misconduct.

At stake is not only Mr. Ghosn’s position atop Renault, but also the fate of the company’s longstanding alliance with Nissan, which Mr. Ghosn also led. Renault and Nissan are being forced to confront the frictions between them and agree on new leadership for their unique auto-making union.