While some of that "uncompensated care" would later be recouped by the hospitals from federal and state government programs, "some would be a loss to the hospitals," the report said.
The Supreme Court is expected to rule on the case in late June.
Ascension Health, the largest nonprofit and Catholic hospital operator in the U.S., sees the case as "a make-or-break moment for the Affordable Care Act," according to Ascension spokesman Nick Ragone. Ascension operates in 23 states and the District of Columbia; 18 of the states are HealthCare.gov states.
"We certainly hope that the Supreme Court rules in favor of those critical subsidies," Ragone said.
The existence of the subsidies, which are available to people with low and middle incomes, Ragone said, has "definitely helped us serve the poor and vulnerable, which is our primary concern in our market. It has helped us serve more people with health care, which is our mission."
The subsidies have also helped the bottom lines of Ascension and other hospital operators.
Ascension, in the second quarter ended Dec. 31, saw a 4.5 percent increase in net patient revenue due to a "favorable change in payor mix," meaning that private insurance plans and Medicaid are picking up a bigger share of patient bills, while the rate of self-paying patients has decreased, Ragone said.
At the same time that Obamacare subsidies have boosted private insurance payments to Acension, the federal government has been cutting the reimbursement rates for Medicare payments. Those Medicare cuts would continue even if the Supreme Court gets rid of the HealthCare.gov subsidies.
"We're feeling pressure from both sides" as a result of the court case, Ragone said.
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HCA, the largest for-profit hospital operator in the nation, cited that pressure to its own bottom line in a brief filed with the Supreme Court asking that the subsidies be upheld. Nearly 9 out of every 10 HCA facilities is located in a HealthCare.gov-served state.
Using strong language, HCA slammed the plaintiffs' claim that Congress meant to deny subsidies to residents of states that failed to establish their own exchange as an incentive to get them to create such marketplaces on their own.
The plaintiffs' claim hinges on language in the ACA that explicitly authorizes such subsidies to customers of state-established exchanges; the law does not use such language when talking about customers of a federally run exchange. The Obama administration rejects that conclusion, saying Congress intended for the subsidies to be issued for HealthCare.gov plans and that the law as written does in fact authorize such aid.