A trial that could determine whether the two largest supermarket chains in the United States can merge opened in Portland, Ore., on Monday, pitting the grocery giant Kroger against regulators who argue that its takeover of Albertsons would eliminate competition at the expense of consumers and workers.

Before Judge Adrienne Nelson of U.S. District Court, the Federal Trade Commission and the supermarket chains laid out their arguments in court for the first time, as union representatives and workers protested the deal on the courthouse steps. Less competition, the agency’s lawyers said, would give Kroger more leverage to raise prices on millions of consumers.

The highly anticipated proceedings, set to last three weeks, come as high food prices have become a critical focus in the presidential race. Vice President Kamala Harris, the Democratic presidential nominee, has backed a federal ban on price-gouging in the food and grocery industries to combat high grocery costs.

Kroger and Albertsons defended the $24.6 billion deal, which would be the biggest supermarket merger in U.S. history, saying it would bolster their leverage with suppliers and improve competition against major retailers like Costco, Amazon and Walmart. But the F.T.C. — backed by a chorus of unions, consumer advocates, politicians and independent grocery chains — reiterated its position that the merger would probably result in higher prices for groceries and worse conditions for workers.

The deal “would eliminate the competition that shoppers and workers depend on in one fell swoop,” Susan Musser, the F.T.C.’s chief trial counsel, said in her opening statement. “This lawsuit is part of an effort aimed at helping Americans feed their families.”

In bringing the case, the F.T.C. has been joined by the attorneys general of eight states, including California and Illinois, as well as the District of Columbia. It’s part of a regulatory push under the Biden administration to rein in corporate consolidation in an array of industries, including airlines, Big Tech, book publishing and pharmaceuticals.