But these combinations will create more than large chains of hospitals. The hospital systems already include medical clinics, employ doctors and provide services ranging from imaging to care at home. Dignity and Catholic Health Initiatives say their proposed deal would create a system that would consist of 139 hospitals, more than 700 sites of care and employ more than 25,000 doctors and other clinicians. The two systems will have annual revenue of roughly $30 billion a year.

The mergers allow these systems to become much larger “and have much stronger tentacles into the patient population they are trying to reach,’ said W. Kenneth Marlow, a health care lawyer with Waller Lansden Dortch and Davis.

The Affordable Care Act masked some of those underlying challenges facing hospitals by supplying a new source of insured patients, and the relative lull in merger activity since the law took effect reflected better financial footing. But the industry is now back to looking at shrinking profit margins and a decline in their core revenues. The Republicans’ proposed tax overhaul could make it even worse by forcing cuts to government programs like Medicare and Medicaid.

“Coming together will allow us to be better prepared to weather the storms,” acknowledged Jim Skogsbergh, the chief executive of Advocate Health Care, which had been foiled by antitrust officials in its earlier attempt to merge with another Chicago-area health system before deciding to combine with Aurora Health Care.

The changing industry dynamics have also caused some of the nation’s largest chains of for-profit hospitals, like Tenet Healthcare and Community Health Systems, to struggle. In addition to shifting their focus to outpatient care, those groups have been shedding some of their weakest hospitals.

In talking about the most recent mergers, much of the reasoning sounds familiar, including the promises around how being bigger will allow the hospital systems to achieve cost savings. Dignity and Catholic Health Initiatives, for example, estimate about $500 million in efficiencies through their merger, and many of the groups point to a larger scale being necessary to pay for the sophisticated computer systems needed to better oversee patients.

But many point to the promises of past mergers as reason to doubt whether the hospital mergers allow much more than an ability to demand higher prices from insurers. After the last wave of mergers that took place a few years ago, the hospitals didn’t use that opportunity to bring their costs down, said Bret Schroeder at PA Consulting Group. They “still aren’t that much more efficient than they were,” he said.