After all, Mr. Trump and his aides have publicly criticized the AT&T-Time Warner deal — the president said last November that it’s “not good for the country.” And he regularly lambastes CNN, the news network owned by Time Warner.

But he’s all praise when it comes to 21st Century Fox and its executive chairman, Rupert Murdoch. In December, the White House press secretary, Sarah Huckabee Sanders, told reporters that the president congratulated Mr. Murdoch on the impending Disney deal. In addition, Mr. Trump praises Fox News and its hosts every chance he gets — and they regularly return the favor. While Disney will not acquire Fox News or the Fox network and stations as part of this deal, the acquisition will make the Murdoch family the largest individual shareholders in Disney, increasing their wealth by billions of dollars.

The Justice Department’s antitrust chief, Makan Delrahim, will surely argue that the president’s feelings about CNN and Fox News have no bearing on his decisions. But it is mystifying why Mr. Delrahim took such a hard line against AT&T-Time Warner, which legal experts argued would be a difficult case to bring because of the nature of the merger, while going so easy on Disney-Fox.

By acquiring Time Warner, AT&T was seeking to buy a supplier — a classic vertical integration move that is hard to challenge because it involves businesses in different parts of an industry. In recent decades, judges have typically concluded that such combinations generate efficiency gains that more than offset any harm they might cause to consumers. That’s why regulators have tended to approve these deals as long as the merging companies agree not to take unfair advantage of their market power.

By comparison, antitrust regulators and judges are usually much more dubious of horizontal deals like Disney-Fox. In these cases, it’s much easier to show that the combined company would have the power to raise prices and limit choices. In the movie business, for example, Disney already wields considerable clout — its studios accounted for more than a third of box office sales in the first five months of the year. The additional 15 percent share of box office sales that the company will gain through this deal no doubt will increase Disney’s clout when it negotiates with movie-theater chains. For instance, the company might be able to demand top billing for its movies and a bigger share of revenue than smaller studios get. According to a handful of theater owners who talked to The Wall Street Journal last year, Disney has already engaged in such tactics, forcing them to accept more onerous terms if they wanted to show its blockbuster “Star Wars: The Last Jedi.”