(Bloomberg) -- Obamacare is under legal attack again,and in the health-care industry it’s hospitals that have the mostto lose.

A federal judge ruled Thursday that subsidies for patients’out-of-pocket costs may be illegal in President Barack Obama’ssignature health-care law, the Patient Protection and Affordable CareAct.

Those subsidies are a key part of the law that helped millionsof Americans afford health care. If the ruling is upheld by highercourts, billions of dollars in payments to hospitals would be atrisk as insurance premiums and out-of-pocket medical costsrise.

Unlike health insurers -- which have pulled out of PPACA in markets wherethey made losses -- hospitals have no such option: They are stuckwith unpaid bills.

“The hospitals are the ones that really stand to lose the mostfrom any potential repeal of Obamacare,” said Jason McGorman, aBloomberg Intelligence analyst who follows the industry. “They haveto treat the patients no matter what.”

Read: 2,000 doctors call for singlepayer

The Affordable Care Act has been under near-constant legalassault since it became law in 2010. While it will take monthsbefore the latest case is resolved by higher courts, shares of thetop three biggest publicly traded hospital companies plungedThursday in response to ruling.

Tenet Healthcare Corp. dropped 9.8 percent, CommunityHealth Systems Inc. declined 11 percent, and HCA Holdings Inc., thelargest U.S. for-profit hospital chain, fell 3.2 percent.

The drops were a setback after a recovery in hospitals stocksthis year following a slump in the second of half of 2015, partlydue to concerns that benefits from Obamacare were waning.

Revenue at risk

Along with the risk of more unpaid patient bills -- whichhospitals count as unlikely-to-be-collected bad debt or charitycare -- the loss of subsidies would also discourage patients fromseeking care, according to Mitchell Morris, global health-caresector leader at the consulting firm Deloitte.

“If it isn’t an emergency, you’re probably not going to have ittaken care of,” Morris said. “I could see decreasing revenue, andincreasing charity care and bad debt.”

Although past cases have ultimately been decided in Obamacare’sfavor, Thursday’s ruling by U.S. District Judge Rosemary Collyerwas a victory for the Republican-controlled House ofRepresentatives, which brought the case. It focuses on whetherthe Affordable Care Act, or ACA, contained a properlegislative appropriation for funds to reimburse insurers forreductions they provide to members in cost-sharing payments:deductibles, co-pays and co-insurance. About 5.6 million people, ormore than half of those enrolled in ACA plans, received thesubsidies last year, according to America’s Health Insurance Plans,an industry group.

The White House said Thursday that the administration isconfident in its legal arguments and blamed Republicans for usingthe court system to resolve political disputes. A decision on thecase is likely months away -- perhaps in the second quarter of 2017-- and a Supreme Court appeal is possible, according to BrianWright, an analyst with Sterne Agee CRT, who called the hospitalsell-off an “overreaction.”

Low-income patients

“This decision does not affect the status quo at this point,”said Chip Kahn, president of the Federation of American Hospitals,an industry trade group. “The millions of low-income patients whodepend on coverage through the Affordable Care Act will continue todo so while this case works its way through the court system.”

Tenet declined to comment. HCA and Community Health didn’treturn calls seeking comment.

The real impact of the ruling is in the uncertainty it injects,said Michael Abrams, a managing partner with Numerof &Associates, a health-care management consulting firm.

“It’s another blow to the viability of the concept as far asimproving access,” he said. “What this does is take a battering ramto the whole idea.”

Insurers exit

Health insurers have the option to leave Obamacare’s statemarkets. UnitedHealth Group Inc. has indicated that will pull outof at least 26 of the 34 state ACA markets it sold plans in forthis year, and Humana Inc., a small player in the program to beginwith, is exiting states, too.

Some hospitals have a great deal hanging on Obamacare, which hashelped drive growth in their Ebitda -- or earnings before interest,taxes, depreciation and amortization.

If the House suit succeeds, “the fear is that the insurers wouldwring their hands and walk away, and a lot of that Ebitda forhospitals would go away,” Bloomberg Intelligence’s McGormansaid.

Read: More coverage on PPACA

Copyright 2018 Bloomberg. All rightsreserved. This material may not be published, broadcast, rewritten,or redistributed.