But last summer, the federal appellate circuit for the District of Columbia jolted the Obama administration with a 2-1 vote saying the subsidies for HealthCare.com enrollees were illegal.
That ruling threatened to yank the subsidies for nearly 5 million enrollees on HealthCare.gov, and leave them facing monthly premium payments that on average would be around 76 percent higher than with the tax credits.
The ruling was effectively overturned—for the moment—because of the decision to have the entire judicial lineup of the D.C. appeals circuit rehear the case. But the U.S. Supreme Court is weighing a request that it take up the issue, raised by another caseand resolve the dispute once and force all.
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The Supreme Court could decide whether to take that second case by the end of October. But the insurers' plans will be in effect by January, months before the high court would be likely to rule. However, if the court did take the case, it could conceivably rule the subsidies were illegal during the same year the plans are in effect.
The language in the new contract due Wednesday recognizes the possibility of the Supreme Court invalidating the subsidies.
"CMS acknowledges that [a health plan issuer] has developed its products for [HealthCare.gov] based on the assumption that [subsidies for premiums and out-of-pocket expenses] will be available to qualifying Enrollees," the contract says. "In the event that this assumption ceases to be valid during the term of this agreement, CMS acknowledges that Issuer could have cause to terminate this Agreement subject to applicable state and federal law."
Several big health insurers asked about the language by CNBC, including Cigna, WellPoint and Independence Blue Cross, referred questions to the insurance industry trade group America's Health Insurance Plans.
Clare Krusing, an AHIP spokeswoman, said only, "This provision just recognizes that health plans' exchange products are based on current assumptions relating to premium tax credits and cost-sharing reductions."
A CMS spokesman had no comment.
Dan Mendelson, CEO of the Avalere Health consultancy, said, "My view is that the insurers participating in the exchange have to prepare for all eventualities. So it's not surprising that they have thought this through."
Mendelson added, however, that the "chances that a drafting inconsistency brings the whole bill down are very small."
"I don't think anyone believes otherwise," he said.
And the language of the clause suggests that insurance plans will have to continue providing coverage to their customers who signed up for 2015 under state and federal law.
But whether many of those customers would be able to afford that coverage without the subsidies remains a large open question.