Renault’s lineup is getting on in years, too. The two companies could share the cost of designing new models, which can easily cost billions of euros.

With Nissan they could rule the world

Renault and Nissan, partners for two decades who added Mitsubishi in 2016, are already in the same league as Volkswagen and Toyota in global car sales. If Jean-Dominique Senard, the chairman of Renault, can hold together the alliance while also merging with Fiat Chrysler, the combined entity would dominate the planet. The carmakers could dictate terms to suppliers, carpet the globe with dealerships, and outspend everyone else on new technologies.

But there is also a less rosy scenario. Which brings us to why the merger might be a bad idea.

They could kill the Renault Nissan Alliance

Renault’s relations with Nissan have been prickly, to say the least, since November after the Japanese authorities arrested Carlos Ghosn, the longtime chairman of the alliance, on suspicion of financial wrongdoing. He has denied all charges.

Since then Hiroto Saikawa, Nissan’s chief executive, has agitated for more autonomy from Renault. He reacted warily to the proposed merger with Fiat Chrysler, which he learned about only days before it was announced.

After a meeting in Yokohama on Wednesday for members of the alliance, the group issued a brief statement that the carmakers had “an open and transparent discussion” on the merger proposal as well as other matters.

Mr. Saikawa said that the merger could ultimately be beneficial, but that he needed “to closely examine it from Nissan’s perspective,” the economic daily Nikkei reported.