There are political consequences, too. As doctors move from being employers to employees, their politics often take a leftward turn. This helps explain why the American Medical Association — long opposed to health care reforms — gave at least a tepid endorsement to Mr. Obama’s overhaul effort.

Gordon H. Smith, executive vice president of the Maine Medical Association, said that his organization had changed from being like a chamber of commerce to being like a union.

Dr. Michael Mirro of Fort Wayne, Ind., is among those caught in the tide. A 61-year-old cardiologist, he began his career like so many of his peers in a small private practice with two other cardiologists. They gradually added doctors until, by last year, they had 22 cardiologists, making theirs one of the largest private heart clinics in Indiana.

But in December, Dr. Mirro and his partners sold everything to Parkview Health, a growing health system that owns the hospital across the street from their building. “We had to hire more and more people to contact insurers and advocate for people to get the care they needed,” Dr. Mirro said. “That’s expensive.” As insurance rates rose and coverage weakened, patients were forced to pay out of their own pockets an increasing portion of Dr. Mirro’s bills. When the economy soured, many stopped trying.

“In the last year, the share of our patients from whom we could not hope to collect any money rose to about 30 percent,” Dr. Mirro said. Dr. Mirro and his partners had been thinking of selling for years. But they made the decision after the Centers for Medicare and Medicaid Services decided last year to cut reimbursements to cardiologists by 27 to 40 percent, depending on the type of practice. The Medicare savings in cardiology are to be used to pay more to primary care doctors, widely seen as under great financial strain.

In the wake of the government decision, cardiology practices across the country began selling out to health systems or hospitals. Dr. Jack Lewin, chief executive of the American College of Cardiology, estimated that the share of cardiologists working in private practice had dropped by half in the past year.

Image Parkview Hospital in Fort Wayne, Ind., part of the Parkview Health system. Such systems have grown rapidly in recent years. Credit... Tom Strattman for The New York Times

“And the remainder of those left are looking to move in that direction,” Dr. Lewin said. “This is all happening with or without reform passing.”