When Judge Richard Leon of the U.S. District Court for the District of Columbia issued an opinion last month green-lighting the massive merger deal between AT&T and Time Warner, he described the weekslong court fight preceding his ruling as a “Herculean task” and “epic battle.”

That battle is about to stretch even further. After weeks of waiting and speculation, the Justice Department last week filed notice to appeal Leon’s ruling to the D.C. Circuit, in hopes of resuscitating federal regulators’ first major legal challenge to a vertical merger in four decades.