The DOJ only endorsed the deal after the carriers agreed to sell parts of their business, including Boost Mobile, to Dish. T-Mobile also has to give Dish robust access to its mobile network for seven years. The idea is for the cable provider to create a new industry player that will replace Sprint, which is known for offering aggressively priced promos to subscribers.

In her letter to Chairman Pai, Senator Klobuchar has raised concerns about what would happen if Dish fails to fill the hole left by Sprint. Her letter -- backed by fellow Senators Ed Markey (D-MA), Tammy Baldwin (D-WI), Tom Udall (D-NM), Cory Booker (D-NJ), Richard Blumenthal (D-CT), Elizabeth Warren (D-MA) and Kirsten Gillibrand (D-NY) -- echoes other critics' misgivings about the merger.

Part of her letter reads:

"As we have previously stated, we have major antitrust concerns regarding the impact of the proposed T-Mobile/Sprint merger on consumers, competition, and the public interest... ...the Consent Decree relies on an uncertain assumption that the harm to competition resulting from the loss of Sprint from the nationwide mobile wireless marketplace would be offset by Dish's replacement of Sprint as a national competitor. The risk posed to consumers if Dish Network fails to compete effectively or build its new wireless business is substantial. Therefore, the public deserves the opportunity to evaluate and comment on this proposal as part of the FCC review process... ...For these reasons, we urge the FCC to issue a public notice and seek public comment on the proposed merger as altered by the Consent Decree and the terms set forth in Dish's extension request letter prior to its consideration of the merger to determine if this transaction is in the public interest. The public should be able to trust that the FCC's process is fair, transparent, and based on a thorough examination of all of the relevant evidence."

Democratic FCC Commissioner Geoffrey Starks tweeted something similar recently, arguing that the commission shouldn't rush its vote without asking for public input given the sheer size and scope of the deal. Whether Senator Klobuchar's letter manages to sway the FCC in a way a multi-state effort to block the merger can't seem to remains to be seen. Based on what a commission spokesperson told The Verge, though, the agency still isn't convinced it should delay its official vote: