The Vermont politician warned in a letter to the department's antitrust division that the combination could result in a "gross concentration of power" in news media, and suggested that blocking the deal would help "preserve our democratic discourse and open competitive markets for speech and commerce."
"This proposed merger is just the latest effort to shrink our media landscape, stifle competition and diversity of content, and provide consumers with less while charging them more," Sanders said.
Read Sanders' letter here:
On Saturday, AT&T and Time Warner jointly announced the deal, unanimously approved by both boards, that will see the mobile company pay $107.50 per share in a cash and stock transaction.
The deal represents a marriage of Time Warner's movie and television empire —including Harry Potter, Batman, Superman and the next generation of super hero movies being developed by DC Comics — with AT&T's wireless network, which covers 315 million people.
The deal, however, faces a stiff political and regulatory test. The populist winds buffeting Washington mean that legislators may not approve of another multi-billion corporate tie-up. For his part, Republican presidential nominee Donald Trump has said his administration would not approve the deal.
—CNBC's JeeYeon Park and Javier David contributed to this report.