Five U.S. senators including presidential candidate Bernie Sanders said they have “significant concerns” that Charter Communications Inc.’s proposed $55.1 billion purchase of Time Warner Cable Inc. could raise prices and thwart competition from cable’s online video rivals.

The deal to create the second-largest U.S. cable provider behind Comcast Corp. would “effectively create a nationwide broadband duopoly” controlling nearly two-thirds of U.S. homes with high-speed Internet connections, the senators said in the Feb. 25 letter to leaders of the Federal Communications Commission and the Justice Department.

Regulators need to act to “prevent any possible harm,” the senators said. Signers included Sanders, a Vermont independent who is running for the Democratic presidential nomination, and Democrats Elizabeth Warren and Ed Markey of Massachusetts, Al Franken of Minnesota, Ron Wyden of Oregon.

Charter is “committed to providing superior broadband and video services at competitive rates,” Tamara Smith, a spokeswoman, said in an e-mail.

The agencies are reviewing the deal proposed in May. With the deal, Charter would almost quadruple its cable subscribers, gaining 12 million customers in cities including New York, Los Angeles and Dallas. The combined business would have about 17 million basic cable customers, compared with Comcast’s 22 million.