WASHINGTON — AT&T and DirecTV told members of Congress on Tuesday that their proposed $48.5 billion merger would be so good for competition that it would do something that has rarely, if ever, happened: pressure cable companies to lower prices.

“Econometric analysis confirms that by making us more competitive, the merger will put downward pricing pressure on cable products — cable bundles, cable video and cable broadband,” Randall Stephenson, chief executive of AT&T, told antitrust subcommittees in both the House and Senate.

AT&T and DirecTV did not promise that they would lower prices, however, and the claim quickly began to dissolve under scrutiny. Pressed by Senator Richard Blumenthal, a Connecticut Democrat, whether AT&T would pass along to consumers all the savings the company expects to realize from the merger, Mr. Stephenson said, “No sir, I cannot commit to that.”

Mr. Blumenthal then asked whether AT&T would commit to passing part of the savings — 75 percent, or maybe 50 percent — to consumers. Not right here and right now, Mr. Stephenson said.