The private Medicare Advantage plans compete not only with one another but also against the traditional fee-for-service Medicare program, through which two-thirds of beneficiaries still receive care, Mr. Bertolini said.

One of the more remarkable moments of the hearing came when Mr. Blumenthal criticized the mergers, even though Aetna, which was founded in Hartford in the 1850s and still has its headquarters there, is a pillar of the state’s economy.

Mr. Blumenthal said he was “deeply troubled” by evidence suggesting that consumers and health care providers would not benefit from the insurance company mergers. He rejected arguments by Mr. Bertolini and Mr. Swedish that new insurance companies were entering the market and creating new competition.

In fact, Mr. Blumenthal said, “there are powerful barriers to entry” for new insurers.

After the hearing, Mr. Blumenthal said that Aetna was “a part of Connecticut’s economic DNA,” a source of jobs and corporate philanthropy. But, he said, “the antitrust analysis is separate and apart” from the company’s role as an employer in Connecticut. He said the merger could adversely affect consumers in Connecticut and other states.

Senator Mike Lee, Republican of Utah, who called the hearing as chairman of the antitrust subcommittee of the Judiciary Committee, said he was determined to find out why “the insurance industry is rapidly consolidating.” Senator Amy Klobuchar of Minnesota, the senior Democrat on the subcommittee, said she wanted to know “whether the claims of consumer benefits, corporate efficiency and lower costs are realistic.”

Mr. Swedish said his company’s merger with Cigna would “uniquely benefit consumers” by expanding access to care through a more extensive network of doctors and hospitals.

He said that the combined company would face “robust competition” in many geographic markets and product lines, including health insurance for individuals and small and midsize employers, and administrative services for big businesses that insure themselves.