The case was filed in a D.C. federal court under the direction of the Justice Department's newly installed antitrust chief, Makan Delrahim. He was only recently confirmed by the Senate and seemed to signal his intentions last week in a speech at the National Press Club in Washington.
Delrahim was critical of the attempts of prior administrations to make "behavioral remedies" conditions to approve of big corporate mergers. Such remedies are supposed to create fairness and protect consumers but create extra burdens on regulators, he said.
The Justice Department has disputed it made the sale of Turner Broadcasting or CNN a condition of approval. A Justice Department official said Monday that the government made a "good faith" effort to resolve the harm to competition while allowing significant parts of the deal to move forward.
Stephenson said during the press conference that a sale of CNN was a "nonstarter," adding that the company was confident it would not have to sell any assets.
Traders have said in recent days that a government move to block the deal could put a chill on merger activity. So far this year, there have been more than 9,000 mergers announced in the U.S., worth an estimated $1.2 trillion, according to Dealogic. Within the U.S. media sector, there have been 209 deals announced this year for an estimated $32 billion.
What's missing this year, however, traders said, were the huge deals that characterized 2016.
While the merger market seemed to awaken since Labor Day, "this could kill it again," said Nancy Havens, an event-driven trader who takes bets on stocks of companies before they complete their deals, about Delrahim's stance. "It makes the future antitrust approval of mergers much more difficult."
AT&T's Stephenson said as much on the conference call. The lawsuit would have "nothing but a freezing effect on commerce."
Justice officials were seen as headed to a lawsuit no matter what. But Delrahim, who has lobbied on behalf of AT&T in the distant past, said in an interview on Canadian television last year he didn't see the deal as having a major antitrust problem.
Last year the Justice Department under the Obama administration teamed up with 11 states to challenge Anthem's $54 billion acquisition of Cigna, and with eight states to block Aetna's $37 billion of Humana. Both deals were canceled earlier this year.