Executives at T-Mobile and Sprint have pitched the merger of their companies as a way for the country to greatly expand its 5G network, a priority for President Donald Trump, who has argued that the widespread adoption of the technology is crucial to national security.

The $26 billion deal seemed to be moving forward in recent weeks, when the head of the Federal Communications Commission gave it his blessing. But on Tuesday, the plan hit a roadblock when a group of state attorneys general sued to block it.

The officials who filed suit, all of them Democrats, said Tuesday that if the merger went through, the prices consumers paid for phone plans would rise as the number of major wireless carries dropped from four to three.

T-Mobile, the nation’s third-largest wireless company, and Sprint, the No. 4 carrier, have insisted they must get bigger to better serve their customers. A merger would reshape the telecommunications industry in the United States and create a formidable rival to the industry leaders, AT&T and Verizon, with each of the three serving roughly a third of the market.

The lawsuit, led by Letitia James of New York and Xavier Becerra of California and joined by eight other attorneys general, was filed in federal court in Manhattan. James said its aim was “to stop the merger in its tracks.”

Even if it does not foil the merger, the lawsuit could delay the deal significantly.

Colorado, Connecticut, the District of Columbia, Maryland, Michigan, Mississippi, Virginia and Wisconsin joined California and New York in filing the lawsuit.