A federal appeals court in the King case ruled last summer that federal subsidies or tax credits given to nearly 5 million enrollees HealthCare.gov, the federal health-care exchange, were legal.



Plaintiffs in that case argued the subsidies are illegal because the Affordable Care Act only explicitly mentions those often-generous subsidies being issued to customers of state-run exchanges. But the Obama administration says the subsidies are legal and proper because of the ACA's intent to help the uninsured afford health coverage.

At the time the Supreme Court said it would hear the case, another virtually identical case was awaiting a re-hearing in the federal circuit appeals court that serves Washington, D.C. There was speculation the Supreme Court would not take up the issue of subsidies unless there ended up being a split between four federal circuits where such cases are winding their way through the system.

The Supreme Court's move comes eight days before open enrollment under the ACA begins Nov. 15.

A decision in the case could come by next June—after people have already re-enrolled or enrolled in coverage with subsidies that make their plans affordable. For now, the subsidies remain in effect.

Both ACA supporters and opponents agree the issue is massive.

Read MoreObamacare: Lower prices are possible



If the subsidies are illegal, it would make insurance unaffordable to many Obamacare customers in 36 states. If too many of them dropped their coverage, it could make continuing the plans financially unfeasible for insurers, because they would be likely to have a disproportionate number of unhealthy enrollees left.

The elimination of the subsidies also would destroy, in those same states, the looming Obamacare mandate that mid- and large-sized employers start offering health insurance to workers in 2015 or face a fine. And it would cripple in those states the "individual mandate," which requires most Americans to have insurance or pay a fine. Both mandates hinge on the availability of subsidies.

Cannon said that while there's speculation the minimum four votes needed to take the case came from the four conservative Supreme Court justices who voted against upholding the ACA in 2012, he "would not be that surprised if some of the Democratic appointees sided with the plaintiffs."

"This case is just that clear cut," he said.

Cannon has long argued the language in the ACA is so clear that there is no question that financial assistance can only be given to enrollees on the 15 exchanges run by individual states and the District of Columbia. Because of that language, the Obama administration relied on regulations issued by the Internal Revenue Service to formally state that subsidies could also go to customers of HealthCare.gov.