Cantwell said the proposal would "strengthen media consolidation and strip newspapers and TV and radio stations of diverse voices."
She threatened that if the FCC moves ahead with the planned changes, Congress could pass a resolution of disapproval, which would repeal the order.
The Senate voted against the FCC's attempt to loosen media ownership rules in 2008 during the George W. Bush administration. A court eventually threw out those changes.
"I'm confident we will push forward in Congress to do the same thing again," Cantwell said.
Sanders and Cantwell both expressed concern that the move would reduce the few number of women and minorities who own media outlets.
The FCC announced earlier this week that it would accept more comments on the proposal, postponing a vote until at least January.
In a statement earlier this week, Bill Lake, the FCC's media bureau chief, insisted the proposed order would not make it easier to own a top TV station and major newspaper in the same market.
"In fact, the order would strengthen the current rule by creating an express presumption against a waiver of the cross-ownership ban to allow such a combination," he said. "In addition, the proposed order preserves the existing TV duopoly rule, which forbids ownership of more than one of the top four TV stations in any market.”