Gov. Gavin Newsom on Wednesday signed a bill giving California’s attorney general and local district attorneys the power to sue individual companies for anticompetitive conduct, ending one of the most combative legislative fights of the year.

The measure, Assembly Bill 1776, known as the Compete Act, updates the Cartwright Act of 1907, which reaches only anticompetitive conduct carried out by two or more businesses. Assembly Member Cecilia Aguiar-Curry, a Davis Democrat, introduced the bill, and progressive Democrats concerned about corporate consolidation in health care, ticket sales and retail signed on as co-authors.

For some of the groups that helped write the bill, the victory is hollow. The American Economic Liberties Project, which spent years helping shape the legislation, dropped its support after Ms. Aguiar-Curry removed a “private right of action” that would have let any person or business sue a company over alleged anticompetitive harm.

Lee Hepner, the project’s senior legal counsel, wrote on X that the governor’s signing message left him pessimistic the law would work. He predicted “politicized antitrust litigation budgets” and “public officials caving to the concentrated private power that antitrust laws are supposed to put in check.”

The private right of action was the state Chamber of Commerce’s biggest objection. The group argued it would “expose businesses of all sizes to a wave of frivolous lawsuits” and ran a multimillion-dollar ad campaign over the summer to weaken the bill. Tech companies including Meta and Google also spent hundreds of thousands of dollars lobbying legislators on the bill and other issues.

Ms. Aguiar-Curry said she was disappointed that the provision was gutted in the final weeks of the session, but she pressed on, and lawmakers passed the bill in its last days. “California now has stronger tools to protect our small businesses, workers, and consumers and to make sure our markets work for everyone,” she said in a release on Wednesday.

Unions and consumer groups backed the measure, among them TechEquity Action, a progressive group that lobbies for regulation of the tech industry. Lorena Gonzalez, president of the California Federation of Labor Unions, said in a statement that the law “gets us one step closer to building a more affordable economy for working people.”

Mr. Newsom signed the bill along with six other small-business measures on the constitutional deadline for acting on legislation. “We’re taking on predatory practices that drive up costs and shut entrepreneurs out — making sure California’s economy works for everyone, not just the biggest and best-connected,” he wrote in a release announcing his approval.

His signing message on the antitrust bill itself was cooler. “While I align myself with a stated goal of targeting anti-competitive conduct that harms consumers, workers, and businesses alike, we must be careful not to set the bar too low,” he wrote, warning against “dragging legitimate, superior business practices and products into the ambit of anti-competitive behavior.” He said he expects judges and prosecutors to apply the law “in ways that penalize clear wrongdoing, without creating needless uncertainty.”

The bill grew out of a three-year review by the California Law Revision Commission, which the Legislature had asked to study changes to the 1907 law.