Federal regulators that want to let T-Mobile complete its acquisition of rival wireless carrier Sprint are pushing back on a collective effort by some states to block the deal.

The $26 billion transaction was subject to federal approval by both the Department of Justice and the Federal Communications Commission. The agencies both blessed the deal, with the DOJ reaching a settlement in July and the FCC granting a green light in October.

The deal also requires approval by regulators in several states, however. While about a dozen have in some way approved the deal or signaled support for the federal settlements, attorneys general representing 13 states and the District of Columbia filed suit to block the merger.

The FCC and DOJ on Friday submitted a filing [400-page PDF] in that case arguing that the deal is in the best interest of the US, and any nationwide injunction holding up the merger would block "substantial, long-term, and procompetitive benefits for American consumers."

The argument, in large part, boils down to: trust us, we're the experts. "Both the Antitrust Division and the FCC have significant experience and expertise in analyzing these types of transactions and do so from a nationwide perspective," the agencies write. "Thus, their conclusions that the merger as remedied is in the public interest deserve appropriate weight in this remedy inquiry by this honorable court."

The dream of 5G competition

The DOJ's Antitrust Division did find that the deal, as structured initially, would "substantially lessen competition in the retail mobile wireless service market," by reducing a field of four players down to three. The regulator and the companies, however, agreed on a novel solution: propping up Dish Network as a new entrant into the market to replace the lost competition. At best, however, it would take Dish several years to spin up a nationwide network matching anything like Sprint's current offerings.

The big upside from the deal, however, is a theoretical boost to nationwide 5G deployment. Under the terms of the agreement with the FCC, the new and improved T-Mobile is required to cover 97 percent of the US population with 5G service within three years, and 99 percent within six years. Said 5G network is also required to provide download speeds of at "least 100Mbps to 90 percent of the US population" within six years.

That said, all the carriers, including T-Mobile, are already working on much-ballyhooed 5G rollouts—none of which exactly lives up to the hype. AT&T will be offering "5G" networks any day now that provide roughly equivalent speeds to its existing 4G LTE service. Verizon just launched 5G coverage maps that basically show the service covering a handful of blocks in the cities where it operates. And T-Mobile itself earlier this month touted the launch of a nationwide 5G network that actually fails to cover about 40 percent of the nation.

In the past month, two different market analysts increased their forecast odds that the states could win the legal case and significantly hamper or block the deal. Firm Raymond James now gives T-Mobile and Sprint a 55 percent chance of prevailing, down from 85 percent earlier. An analyst for Cowen research, meanwhile, gives the companies only a 40 percent chance of a win.