Time Warner owns CNN, and the president has at times taken aim at the news network for its coverage of his administration. During his presidential campaign, he said the merger should be blocked.
Antitrust regulators are required to make decisions independently of the White House. Mr. Delrahim has denied any interference in the decision to block the merger.
During the pretrial discovery period, the parties notify the court of requests such as witness lists, documents and interviews. A judge will intervene only when there is a dispute over the demands.
AT&T has amassed a considerable army of litigators and more than 100 lobbyists to win its case. Its top litigator, Daniel M. Petrocelli, defended Mr. Trump in a lawsuit involving Trump University. Time Warner has hired Christine A. Varney, the former head of the Justice Department’s Antitrust Division.
The decision by Mr. Delrahim, a former White House counsel to deny a merger of two companies that do not directly compete was unexpected given that similar deals have been approved. The Department of Justice has argued that the merger of two giant companies that both produce content and distribute those shows will lead to higher prices for consumers.
AT&T has claimed, however, that it appeared that Mr. Delrahim had unfairly singled out its deal. As evidence, it pointed to a decision in 2011 by the Department of Justice to approve a similar deal, Comcast’s merger with NBCUniversal.
Mr. Delrahim has insisted that Comcast has a much smaller reach into American homes compared with AT&T’s nationwide wireless and satellite television network.
Before he was appointed to the Justice Department, Mr. Delrahim said in an interview with a Canadian broadcaster that he did not see clear problems with the deal.
The strategy of putting Mr. Delrahim on the witness stand has its risks.
“If AT&T’s strategy is to call Delrahim to testify for the purpose of showing political motivation, it could blow up in their face if the assistant attorney general is a credible witness for the legitimate challenge of the merger,” said Gene Kimmelman, a former senior antitrust official in the Justice Department who has been critical of the merger. “Unless they have a smoking gun, I think Delrahim has a convincing argument.”
An earlier version of this article misstated the year when the Department of Justice approved Comcast’s acquisition of NBCUniversal. The year was 2011, not 2013.