Franken: 'I don't trust promises' of Comcast, NBC given past experience

In the past, Franken said, NBC made promises to allow programming it did not own to air on its stations. But instead, it is common practice for networks to demand ownership of independent programming before placing it in primetime lineups.

"It's completely opposite of what NBC said they would do," he said. "You'll have to excuse me if I don't trust these promises."

Comcast CEO Brian Roberts pledged NBC content will be made available to competitors and to consumers online.

Franken said he is also worried this merger will set off another round of media consolidation, similar to when Viacom bought CBS and Disney bought ABC.

"The next thing we know is AT&T and Verizon think they have to buy a network in order to compete," he said.