Sen. Al Franken, D-Minn. (Manuel Balce Ceneta/AP)

In a filing sent to the Justice Department and Federal Communications Commission, Franken said the deal would lead to a market duopoly and that conditions attached to the merger wouldn’t stop what could be as much as a 25 percent increase in wireless costs for consumers.

“The competitive effects of a merger of this size and scope will reverberate throughout the telecommunications sector for decades to come and will affect consumer prices, customer service, innovation, competition in handsets, and the quality and quantity of network coverage,” said Franken, a member of the Senate Judiciary Committee. ”These threats are too large and too irrevocable to be prevented or alleviated by conditions.”

Franken’s move comes after committee chairman Herb Kohl (D-Wis. wrote a letter to federal officials last week, saying such a merger would violate antitrust law.

On the flip side, more than six dozen lawmakers have expressed support for the merger as have several high-tech giants such as Microsoft and Facebook.

Franken said conditions wouldn’t stop Verizon, which would benefit from the concentration in the wireless market, from anticompetitive practices.

Specifically, Franken rejected AT&T’s assertion that it needed more spectrum from T-Mobile to better serve customers. He said AT&T has more spectrum than any other wireless company but hasn’t used the airwaves to alleviate all the dropped calls its iPhone customers have complained of in recent years.

He said the promise of divesting spectrum as a condition to approval doesn’t help, either, because there may not be any wireless companies in a position the buy those assets.