At a hearing this summer about the rising power of the country’s biggest tech companies, Representative David Cicilline zeroed in on Amazon. Unhappy with a response from one of the company’s top lawyers, he delivered a biting retort.

“I may remind you, sir: You are under oath,” Mr. Cicilline said.

Tech companies are under various antitrust investigations, including by the Justice Department, the Federal Trade Commission and state attorneys general. Those inquiries could lead to lawsuits against the companies to enforce existing laws.

But Mr. Cicilline has a more ambitious goal — one that may be the greater threat, in the long run, to Big Tech’s practices and profits. He’s trying to build evidence, and a bipartisan consensus, for changing the laws themselves.

When his party took control of the House this year, Mr. Cicilline, a five-term Rhode Island Democrat, became chairman of the subcommittee that oversees antitrust law. In June, he opened an investigation into possible anticompetitive practices by Google, Facebook, Apple and Amazon.
Under his direction, the panel has held hearings and collected thousands of documents from the companies. The lawmakers have requested countless more records, including emails among top executives.

He has made clear to the companies that he is not messing around. When he found the July testimony of industry witnesses incomplete, Mr. Cicilline sent the companies follow-up questions. His list: 181 questions for Google, 45 for Facebook, 158 for Amazon and 43 for Apple.

“We’re the only group,” the 58-year-old lawmaker said in a recent interview, “that has the ability to actually propose the solution.”

Antitrust experts say this is the most serious congressional inquiry into potential anticompetitive corporate behavior in decades. Among the people at Mr. Cicilline’s side are Slade Bond, the subcommittee’s top lawyer, and Lina Khan, a young antitrust scholar whose writing makes the case for curbing Big Tech’s market power.

“There’s a long game being played here — interrogate, educate and prepare for antitrust reform,” said Herbert Hovenkamp, an antitrust expert at the University of Pennsylvania Law School.

Mr. Cicilline seems an unlikely inquisitor of Big Tech. He is a criminal defense lawyer turned politician — in the Rhode Island General Assembly, as mayor of Providence and then in Congress. He has no track record in antitrust.

But people familiar with Mr. Cicilline are not really surprised by his recent turn. Darrell West has closely followed Mr. Cicilline’s career since the early 1980s, when Mr. West was a young assistant professor of political science at Brown University and Mr. Cicilline was his student.

The political scientist’s report card on Mr. Cicilline: intelligent and hardworking with grade-A political instincts.

His sure sense of political timing was notably evident when Mr. Cicilline ran to unseat Vincent A. Cianci Jr., the longtime Providence mayor known as Buddy. Mr. Cicilline announced his candidacy before Mr. Cianci was indicted on racketeering charges, Mr. West recalled. Mr. Cicilline was elected to the first of his two terms as mayor in 2002, while Mr. Cianci went to federal prison.

“I’ve always been impressed by how canny he is at sizing up the moment,” said Mr. West, director of governance studies at the Brookings Institution. “We’re seeing that now again.”

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.
The congressional antitrust revival is not just a preoccupation of Democrats like Mr. Cicilline. Republicans also say it is time for scrutiny of the tech companies.

In September, when the House team sent lengthy information requests to the chief executives of Google, Facebook, Amazon and Apple, the letters bore the signatures of four congressmen: Mr. Nadler, the chairman of the Judiciary Committee; Doug Collins of Georgia, the ranking Republican on Judiciary; Jim Sensenbrenner of Wisconsin, the ranking Republican on the antitrust subcommittee; and Mr. Cicilline.

At the hearings, the comments and questions from the Republican and Democrats have shown divisions. The Republicans emphasize that being big is not being bad and that success should not be punished. The Democrats speak of competitors stifled and consumers exploited.

They do share concerns. “These are some of the largest and most powerful companies in the world, and it’s important for us to understand the dynamics of this new industry,” Mr. Collins said in an interview.

Specifically, he said, the tech giants should be restrained from buying up nascent competitors.

In their investigations of Facebook, the F.T.C. and the states are examining that issue. Did the social network buy Instagram, the photo-sharing service, and WhatsApp, the messaging service, as steps in a strategy to squelch emerging rivals?

Limiting the freedom of large tech companies to acquire upstart competitors, legal experts say, would require modifying the antitrust laws.

In the House hearings, the topic of “predatory acquisitions” has come up repeatedly. Another idea discussed by some expert witnesses at the House hearings is the creation of a new regulator empowered to respond more quickly to allegations of anticompetitive behavior.
Antitrust traditionalists regard both proposals with skepticism. The changes would be a shift from the current enforcement model, in which courts rule on individual cases. Instead, government officials would need to predict the path of innovation, said Maureen Ohlhausen, a former F.T.C. commissioner who testified at one of the hearings led by Mr. Cicilline.

“That’s what markets are good at, not government,” said Ms. Ohlhausen, a partner at Baker Botts, echoing the view of the tech companies and free-market adherents.

Mr. Nadler, Mr. Cicilline’s boss as head of the Judiciary Committee, was reluctant to say where the inquiry would lead. He called the investigation “the beginning of a long process.”

Not even Mr. Cicilline is sure where it will go.

“It’s fairly easy to identify the challenges,” he said. “Crafting the solutions is much more difficult.”

But he is glad that he agreed to Mr. Nadler’s recommendation.

“It turned out, of course, to be a really good decision,” Mr. Cicilline said.