Back in 2015, Leonard De Monte was feeling settled. At 31, he had health insurance and was making a union wage at the Vons grocery store in Woodland Hills, Calif., where he had worked for more than a decade. A familiar face in the bakery section, he knew dozens of frequent shoppers’ orders by heart.

Then came a corporate merger: Albertsons, Vons’s parent company, acquired its rival Safeway. Mr. De Monte’s store was sold to a third chain as part of the deal, and within months of the change, the store’s new owner declared bankruptcy. Mr. De Monte found himself out of work.

Former customers vouched for him, and he found a new job at a local Pavilions, part of another grocery chain owned by Albertsons. But he had lost his seniority and was demoted to minimum wage.

“All my hard work was flushed down the toilet,” Mr. De Monte said.

Now, nearly 10 years older and having finally worked his way up to a wage of nearly $27 per hour, he’s experiencing déjà vu: Albertsons is trying to merge with Kroger in a $24.6 billion deal that will be the biggest grocery combination in history if it goes through. The two chains have agreed to sell 579 stores — out of about 5,000 — to a third company in an effort to satisfy antitrust regulators. The Pavilions where Mr. De Monte works is on that list.

Mergers often create anxiety for workers who stand to lose jobs or benefits when companies combine. The United Food and Commercial Workers International Union, or U.F.C.W., which represents most in-store workers at Kroger and Albertsons, has spoken out against the proposed deal, though it doesn’t have much ability to stop it.

But the union does have a powerful ally: the Federal Trade Commission. The agency sued to block the combination, and a trial that will decide whether the two chains can join forces is scheduled to start in federal court in Oregon on Monday.

The F.T.C. is making arguments typical of antitrust enforcers in recent decades: The merger will decrease competition, leading to higher prices for consumers.

But within its legal complaint is another claim, one that has surprised some antitrust experts because of its novelty. The combination of the nation’s two biggest supermarket chains, the F.T.C. argues, would erode the bargaining power of unions and harm not just consumers, but workers as well.

Starting in the late 1970s, after a period of robust antitrust enforcement, regulators eased up on challenging corporate mergers. Regulators under the Biden administration, however, have made cracking down on corporate concentration a priority. And for the first time, merger guidelines updated last year by the F.T.C. and the Justice Department explicitly outline the agencies’ emphasis on how corporate mergers could reduce competition for workers and result in lower wages or worse benefits.

“Recognizing that there’s a web of intersecting harm that can happen is an extension, in my mind, of the underlying principles of antitrust enforcement,” said Christine Bartholomew, a professor at the University at Buffalo School of Law who teaches antitrust. “The pendulum is swinging back to recognize the broader types of harm from anticompetitive conduct.”

The attorneys general of Colorado and Washington State, who have separately sued to block the supermarket deal, also centered workers in their complaints.

The grocery industry has seen waves of consolidation since the 1990s. Now just four companies — Walmart, Kroger, Costco and Albertsons — account for about half of all grocery sales.

Kroger and Albertsons collectively employ about 700,000 people. The new corporation would operate under the Kroger name, and a Kroger spokeswoman said all frontline workers would keep their jobs and existing union contracts. But Mr. De Monte is not convinced that his job and benefits would be guaranteed, or that the chain buying his store would keep it open.

His wounds from the last merger are still fresh.

A Regulatory Shift

The F.T.C.’s position today looks very different from the one it took in 2015. Back then, the regulator approved the merger of Albertsons and Safeway, satisfied that the 146 stores eventually sold to a third party — Haggen — would prevent dominance by a single supermarket chain in certain markets.

The U.F.C.W. did not strongly object to that merger or to the sale of stores, either, something the union came to regret once Haggen filed for bankruptcy and thousands of workers lost their jobs.

This time around, Kroger and Albertsons have proposed a similar solution to gain antitrust approval: selling 579 stores — along the West Coast and in Colorado, Arizona, Illinois and a handful of other states — to a company called C&S Wholesale Grocers. But the F.T.C. is not convinced that separating out about a tenth of the stores would effectively maintain competition or mitigate the harm to workers and consumers.

Although only about 13 percent of grocery store workers are unionized, most of the workers at Kroger and Albertsons are represented by the U.F.C.W.

“I have great health benefits because I’ve been with the company so long,” Mr. De Monte said, adding that he needs regular checkups because of a past cancer diagnosis. “If I lose my health benefits, I would have to pay out of pocket.”

The U.F.C.W. is concerned that the combined strength of Kroger and Albertsons would intensify a power imbalance with the union. John Marshall, a financial analyst for U.F.C.W. chapters in California and Washington State, said that, individually, both chains had been aggressive at the bargaining table. In 2003, they each demanded concessions from the U.F.C.W., including the introduction of a two-tiered pay structure. Despite setbacks, unionized workers at the companies have retained health and retirement benefits that their counterparts at nonunion rivals like Walmart lack.

Kroger has said it needs to merge with Albertsons to compete against Walmart and Amazon. Walmart employs two million people and has been accused of illegal union busting, allegations the company has denied. A Kroger spokeswoman said nonunion rivals would become “even more powerful and unaccountable” if the merger was blocked.

The F.T.C., however, argues that a combined Kroger and Albertsons would erode unions’ ability to negotiate better pay and benefits in bargaining talks.

“The unions that represent grocery workers leverage the fact that Kroger and Albertsons are separate companies competing for customers and workers to negotiate better terms of employment for union grocery workers,” the F.T.C. complaint reads. The deal would “eliminate that competition” and lead to lower wages, worse benefits and weaker worker protections. An agency representative declined to provide additional comment beyond the legal complaint.

Eric Posner, a professor at the University of Chicago Law School who focuses on antitrust, noted that a more dominant Kroger would chip away at unions’ ability to use strikes as a bargaining tool.

“If the worker can find an equally good job elsewhere, then the workers can stay on strike longer, and that means the employer will have to give and make concessions,” Mr. Posner said.

He said he was not aware of any other antitrust cases that limited the scope of harm to unionized workers. And regulators have raised labor-related concerns in only one other case that has gone to court, Mr. Posner added. In that 2022 suit, the Justice Department successfully blocked a merger of book publishers, focusing on authors as workers who stood to be harmed by the deal.

Closings and Layoffs Feared

On top of weakened bargaining power, workers — especially those who experienced the fallout from Albertsons’s takeover of Safeway a decade ago — are concerned about potential store closings and layoffs.

Michael Lawing, a meat manager at an Albertsons in the Seattle area, has been an employee of the company on and off since 1987. He said he and all his colleagues lost their jobs when their store switched over to Haggen ownership in 2015.

“I lost all my seniority as far as vacation time, as far as health benefits,” Mr. Lawing said. “I had to restart from the beginning.”

Kroger has portrayed C&S, which has signed up to buy the 579 stores that would be shed under the merger, as a pro-union operator. Lauren La Bruno, a C&S spokeswoman, said the company would recognize the union work force and honor all collective bargaining agreements.

But Mr. Marshall of the U.F.C.W. said that at two meetings in January, C&S representatives had refused to promise to negotiate new collective bargaining agreements with the union once the current contracts expired. Contracts covering more than 100,000 Kroger and Albertsons workers, mostly on the West Coast, are set to run out next year, he said. Ms. La Bruno did not respond to a request for comment on those meetings.

C&S is primarily in the wholesale grocery supply business and currently operates just 23 supermarkets nationwide, according to the F.T.C. While Ms. La Bruno said the company had enough financial strength and experience in food retailing to operate hundreds more stores, antitrust experts and regulators say another Haggen-style collapse is likely if the deal goes through. They argue that C&S doesn’t appear to be equipped to efficiently operate hundreds of supermarkets.

“This company might just shut down the stores after buying them,” Mr. Posner said.

Yasmin Ashur, who has been an Albertsons employee for nearly 25 years, works as a cashier at one of the company’s stores in Port Orchard, Wash., which is set to be sold to C&S. Her pension and health insurance are top of mind as she thinks about what will happen after the current U.F.C.W. contract expires.

Ms. Ashur earns $26 an hour, about $10 above the minimum wage in her state. She said there were other places she could look for work, if need be — maybe at a nursing home or a discount retailer like Big Lots.

“But then again, you’re starting from scratch,” Ms. Ashur said. “Nobody will hire you for whatever I was making, and I’m going to have to start from the bottom.”