"While we have a great deal of respect for Senator Kohl, we strongly disagree with his analysis of this transaction, which will bring significant benefits to American consumers," Tom Sugrue, T-Mobile's senior vice president of government affairs, said in a statement. "We are confident that the review process will demonstrate that the merger is fully consistent with the antitrust laws and significantly advances the public interest."

AT&T and T-Mobile have said the deal will allow AT&T to roll out mobile broadband services to a larger geographical area. T-Mobile executives have said they don't have the resources or spectrum to offer 4G LTE service, although the company now offers the slower HSPA+ quasi-4G service.

Several minority groups and labor unions have voiced support for the deal.

Kohl questioned AT&T's promises that the deal would allow it to expand its 4G LTE rollout from 80 percent of the U.S. population to 97 percent. Much of the T-Mobile spectrum AT&T would require doesn't cover rural areas, he said.

The expansion of a "new generation of cell phone service not yet deployed six years from now is plainly too speculative and uncertain to justify this merge under antitrust laws," he wrote.