Fiat Chrysler “was able to obtain unique advantages, different and distinct, and in fact denied to G.M.,” Craig Glidden, G.M.’s general counsel, said during a conference call with reporters.

In a statement, Fiat Chrysler said it was “astonished” by the lawsuit and suggested that it was timed to disrupt the company’s contract negotiations with the U.A.W. and a proposed merger with Peugeot SA, the French automaker. “We intend to vigorously defend against this meritless lawsuit and pursue all legal remedies in response to it,” the company said.

G.M.’s complaint, filed in the United States District Court in Detroit, is based on findings from a broad investigation into corruption at the U.A.W. In 2017, federal investigators revealed that Fiat Chrysler’s head of labor relations, Alphons Iacobelli, had allowed union officials to put personal expenses and travel on credit cards linked to a training center funded by the company. Those union officials included General Holiefield, who led U.A.W. negotiations with Fiat Chrysler from 2008 until 2015. Prosecutors said Mr. Iacobelli approved $1.2 million in spending by Mr. Holiefield and his wife.

Mr. Iacobelli used training center funds himself, including to buy a $350,000 Ferrari and pay for renovations at his home in Rochester Hills, Mich., an affluent Detroit suburb. Mr. Iacobelli pleaded guilty in 2018 and was sentenced to five and a half years in prison. Mr. Holiefield died in 2015 before any charges were filed in the case. His wife, Monica Morgan, pleaded guilty in 2018 and received an 18-month prison sentence. Mr. Holiefield’s successor, Norwood Jewell, was sentenced in August to 15 months in prison in connection with the case.

Mr. Iacobelli, and two other Fiat Chrysler executives who pleaded guilty to charges connected to the corruption inquiry, Jerome Durden and Michael Brown, were named as defendants in the G.M. suit. In his plea agreement, Mr. Iacobelli said he authorized $1.5 million in spending by U.A.W. officials in an effort to curry favor from the union in contract negotiations.

G.M.’s lawsuit claims the improper use of union money influenced contract negotiations between Fiat Chrysler and the U.A.W. in 2009, 2011 and 2015. It contends the union allowed Fiat Chrysler to hire more new and temporary workers at entry-level wages than it allowed G.M., giving Fiat Chrysler a cost advantage. The union also helped Fiat Chrysler cut costs further by supporting an overhaul of its manufacturing process, and backed Mr. Marchionne’s plan to merge with G.M., a deal that would most likely have resulted in the loss of thousands of union jobs.

G.M. did not sue the union, which recently reached a new agreement with G.M. after a 40-day strike and is in the middle of negotiating a new contract with Fiat Chrysler. But the union has been thrown into turmoil by the broader federal investigation of corruption by its top officials. The cases brought by the Justice Department have angered many union members who feel that some of their leaders have forsaken workers’ interests and are only looking out for themselves.