Sen. Al Franken (D-Minn.), already a critic of Comcast’s proposed merger with Time Warner Cable, is warning Justice Department officials that the combination “could compromise the open nature of the Internet.”

Franken sent a letter to Renata Hesse and David Gelfand, the two deputy assistant attorneys general in the Antitrust Division of the Justice Department who are scrutinizing the proposed merger, unveiled last month.

“Simply put, the Internet belongs to the people, not to huge corporations,” Franken wrote. “Comcast’s proposed acquisition of Time Warner Cable could disrupt this balance of power, resulting in higher costs and fewer choices for consumers.”

Franken has so far been the most visible critic in Congress of the merger, and has appeared on CNN and “CBS This Morning” to express his concern that the combination of the No. 1 and No. 2 largest cable providers will be bad for consumers. He is a member of the Senate Judiciary Committee, which has scheduled the first hearing on the transaction on April 2.

“I am very concerned that Comcast could use its clout in the broadband market to dictate the content consumers receive and the prices they pay, and these concerns are only intensified by Comcast’s proposal to acquire Time Warner Cable,” Franken wrote. “With more than 20 million customers, Comcast already is the nation’s dominant Internet service provider, controlling about a quarter of the national broadband market and a much higher percentage of the market in many of the local areas in which it operates.”

Under the conditions it agreed to as part of its acquisition of NBCUniversal in 2011, Comcast has to abide by the FCC’s net neutrality guidelines until 2018. They must abide by those guidelines even though an appellate court struck down anti-blocking and anti-discrimination provisions earlier this year.

Franken, however, said that there is still a “question” as to what happens after 2018, and he noted that the net neutrality provisions do not extend to peering arrangements.

His full letter is here.

Comcast issued a statement in response to Franken’s letter, saying that the transaction “will bring millions more Americans under the Open Internet rules as soon as our deal closes. We fully expect that the FCC will have in place Open Internet rules that will apply to all companies by the time our current condition from the NBCUniversal deal expires in 2018. That condition was always meant as a bridge to enforceable rules that would be applicable to all companies in the industry. Comcast has supported Open Internet rules since they were first proposed and is the only company that is currently required to abide by them.”