But the Justice Department sued to block the merger last November. Mr. Delrahim and other officials said the combination of telecom and media giants would harm competition and lead to higher prices for consumers.
The case has also been clouded by politics. Time Warner owns CNN, and President Trump has at times taken aim at the news network for its coverage of his administration. During his 2016 presidential campaign, he said the merger should be blocked.
AT&T has not directly mentioned the potential of political interference by the White House in Mr. Delrahim’s actions against AT&T and Time Warner. But AT&T’s chief executive, Randall L. Stephenson, has publicly questioned the motives of officials at the Justice Department.
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.
The Justice Department has also submitted a list of witnesses, along with other documents from AT&T and Time Warner, said the people. 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.
“We will reserve comment on it for the courtroom,” Daniel M. Petrocelli, lead trial counsel for Time Warner and AT&T, said in a statement on Wednesday, referring to the reports about Mr. Delrahim being put on AT&T’s witness list.
A spokesman for the Department of Justice declined to comment.
AT&T has amassed a considerable army of litigators and more than 100 lobbyists to win its case. Its top litigator, Mr. 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 previously been approved. The Justice Department 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 that Mr. Delrahim unfairly singled out its deal. The company described the government’s decision as “selective enforcement” and as evidence, it pointed to a 2011 decision by the Justice Department to approve a similar deal, Comcast’s merger with NBCUniversal.
AT&T is expected to also claim Mr. Delrahim has flip-flopped on his views of the deal. 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 company may present a video of that interview in the trial.
Mr. Delrahim has said Comcast had a much smaller reach into American homes than AT&T’s nationwide wireless and satellite television network. He has added that Comcast and NBC were allowed to merge with several so-called behavioral conditions that were hard to enforce. Mr. Delrahim has vocally opposed such conditions that constrain a company’s business practices, saying he prefers selling off assets to solve competition concerns.
AT&T’s strategy of putting Mr. Delrahim on the witness stand is risky.
“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 deal. “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.