The French government signaled that it would back Fiat’s plan as long as jobs and industrial sites in France would not be compromised. But the powerful C.G.T. labor union at Renault attacked the proposal on Monday, saying it gave Fiat the power to favor operations in Italy over France, because the French government would no longer have a representative on the new board. At least 22,000 jobs have been shed at Renault in France since 2005, the C.G.T. added.

“Nothing would prevent the Italian shareholder from favoring its national base,” the union said in a statement, noting that it learned about the talks only through the news media. “The C.G.T. considers that there is nothing good to expect from a Renault-F.C.A. merger.”

Matteo Salvini, Italy’s deputy prime minister and leader of the country’s League party, greeted the merger proposal. “Fiat Chrysler expanding is good news for Italy,” he said. “I count on it being a brilliant operation that preserves every single job while creating a European car industry giant.”

Fiat Chrysler and Renault each have things to offer to partly address each other’s weaknesses. Renault and Nissan were among the first car companies to build electric vehicles for mass production, whereas Fiat is seen as a laggard. Together, the companies sold 8.7 million vehicles last year, ahead of G.M., which sold 8.4 million.

Fiat Chrysler also offers Renault access to the United States market, where the French have no presence. Their model lineups complement each other fairly well, with Fiat Chrysler strong in sport utility vehicles like the Jeep Cherokee as well as the Ram pickup truck line. Renault has a broader palette of passenger cars like the compact Mégane or subcompact Clio.

“Putting F.C.A. and Renault together could make sense,” said Ferdinand Dudenhöffer, a professor at the University of Duisburg-Essen in Germany who is a prominent commentator on the car industry.

But neither company is strong in China, which has become the world’s largest car market. And neither is particularly strong in the high end of the car market, the most profitable segment. Fiat’s Alfa Romeo and Maserati brands are well regarded but are minor players in a market dominated by Mercedes-Benz, BMW and Audi.