Senator Herb Kohl, a Wisconsin Democrat and chairman of the Senate Judiciary Committee's antitrust subcommittee, on Wednesday called on the U.S. Department of Justice and the U.S. Federal Communications Commission to reject the deal between the nation's second-largest and fourth-largest mobile carriers.

"I have concluded that this acquisition, if permitted to proceed, would likely cause substantial harm to competition and consumers, would be contrary to antitrust law and not in the public interest, and therefore should be blocked by your agencies," Kohl to the DOJ and FCC.

The deal would reduce the number of national carriers from four to three and would give AT&T and Verizon a combined market share of nearly 80 percent, Kohl said, repeating frequent criticisms of the merger. The deal would eliminate T-Mobile as a low-cost competitor to other carriers, he said.

The changes in the mobile industry caused by the acquisition would be "highly dangerous" to consumers, Kohl wrote.

T-Mobile USA said it was disappointed with Kohl's opposition to the deal.