The Washington attorney general sued Kroger and Albertsons on Monday to block the merger of the two largest supermarket chains in the U.S. He is asking the court to grant a permanent nationwide injunction.

The mega-deal, worth $24.6 billion, promised to shake up competition in the food aisles. Kroger, the biggest supermarket operator with 2,719 locations, owns Ralphs, Harris Teeter, Fred Meyer, King Soopers and other chains. Albertsons is the second-biggest chain, with 2,272 stores, and owns Safeway and Vons. Together they employ about 720,000 people.

Yet Kroger and Albertsons say they must unite to stand a chance against nontraditional rivals, including Amazon, Costco and especially Walmart. The grocers say the latter two companies sell more groceries than Kroger and Albertsons combined. And they emphasize that they offer union jobs, in contrast to the rivals. They had hoped to close the deal in August.

The lawsuit, filed in Washington state court, may throw a wrench in those plans. Attorney General Bob Ferguson argues that, because the two chains own more than half of all supermarkets in his state, their proposed union will eliminate a rivalry that helps keep food prices low.

"Shoppers will have fewer choices and less competition, and, without a competitive marketplace, they will pay higher prices at the grocery store," Ferguson said in a statement.

A legal challenge to the merger does not come as a surprise. The Federal Trade Commission has been reviewing the proposed deal for over a year. Multiple state officials and lawmakers have voiced concerns that the tie-up risks reducing options for shoppers, farmers, workers and food producers. As early as May 2023, Kroger CEO Rodney McMullen said the two grocery chains "committed to litigate in advance" if federal regulators or state attorneys general rejected the merger.

Ohio-based Kroger and Idaho-based Albertsons overlap particularly in Western states. To pre-empt regulators' concerns about diminishing grocery competition in those markets, the retailers found a buyer for up to 650 stores that they'd sell off as part of the merger: C&S Wholesale Grocers, a supplier company that also runs some Piggly Wiggly supermarkets.

Ferguson said that plan does not go far enough to protect supermarket employees and customers in his state. His office asserts the combined Kroger-Albertsons would still "enjoy a near-monopoly" in many parts of Washington. It also questioned whether C&S could run the markets successfully.

Albertsons' merger with Safeway in 2015 serves as a warning in that regard. The FTC required it to sell off 168 stores as part of the deal. Within months, one of its buyers filed for bankruptcy protection and Albertsons repurchased 33 of those stores — some for as little as $1 at auction, Ferguson says.

Antitrust experts in the Biden administration had previously spoken skeptically about whether divestitures sufficiently safeguard competition, including on prices and terms struck with suppliers. The regulators have also pushed for tougher scrutiny of megadeals, making this merger a high-profile test.