After the 2016 election, with Donald J. Trump in the White House and Republicans in the majority in the House, the top Democrat on the Judiciary Committee, Representative Jerrold Nadler of New York, urged Mr. Cicilline to become the ranking minority member on the antitrust subcommittee. Mr. Cicilline said he was reluctant at first. Antitrust had been dormant in Congress for years.

But as they talked, Mr. Cicilline became convinced that the stagnant incomes of middle-class workers and the growing wealth gap in America were at least partly related to an increasing concentration of economic power. He decided to take on the assignment.

Over the next two years, one antitrust topic kept coming up: the market clout of the tech giants. Until then, most of Mr. Cicilline’s experience with tech had been as a consumer. He drives a Tesla, writes his own tweets, and sometimes buys books and movies on Amazon.

“The more I learned, the more alarming it became,” Mr. Cicilline said.

His panel plans to complete its investigation and publish its findings and recommendations early next year. The prospect for legislative action someday hinges on several unknowns. The most significant include what the subcommittee finds, the 2020 election results and the strength of public support for curbing the tech giants.

There is no realistic chance that antitrust legislation will be taken up next year, in the heat of a presidential election campaign. But the House investigation does point to a renewed congressional interest in the economic and social impact of concentrated market power and wealth, after years of neglect.

In the past, congressional inquiries laid the foundation for antitrust reform. Investigations led by Senator Philip A. Hart, a Michigan Democrat, in the 1960s and 1970s paved the way for antitrust actions in industries from telephones to breakfast cereals, and for new legislation that strengthened oversight of corporate mergers. They also led to new powers for the Justice Department.

“That was a major contribution to antitrust enforcement, and today’s House investigation could be an important step in the same direction,” said William Kovacic, a law professor at George Washington University and a former chairman of the F.T.C.