Those deals were known as “horizontal integration” because similar businesses merged. AT&T’s proposed acquisition of Time Warner, however, is considered “vertical” because the two companies largely do not compete against each other but operate on the same supply chain.

Still, regulators may look at other ways AT&T could affect the media ecosystem if the deal is completed. AT&T could make it more expensive for its competitors to gain access to Time Warner’s content or give preferential treatment to its own programming, said John Bergmayer, senior counsel at Public Knowledge, a digital rights advocacy group.

The merger would make AT&T unmatched in its size and reach to consumers through smartphones, home broadband, satellite television and a broad portfolio of cable channels and movies. For that reason, it may raise more cautionary flags than Comcast’s merger with NBCUniversal, which did not involve a wireless carrier.

Many experts in Washington still look unfavorably at the outcome from Comcast and NBCUniversal because the terms of their settlement were too difficult to enforce.

“It was an unsuccessful merger from a regulatory point of view,” said Andrew Schwartzman of the Georgetown University Law Center’s Institute for Public Representation.

In the case of Comcast, some smaller competitors, particularly programmers that compete with NBC, complained that the company did not fulfill promises it made with the approval by the Justice Department and the F.C.C.

The financial information company Bloomberg L.P. complained that Comcast had put its business news channel, which competes with CNBC, in the equivalent of the channel menu hinterlands. Comcast had promised not to discriminate against competing programmers by putting only its own channels in prime locations on the dial next to similar and popular programming. Bloomberg sued Comcast, saying that its channel was put on odd channels, far away from other business news, and that consumers had a hard time reaching it.