Conversely, the Obamacare enrollees who would be more likely to be able to afford the insurance without the subsidy assistance or get coverage from another source after a Supreme Court ruling against the subsidies are more likely to have higher incomes, to be white, highly educated "and to live in regions outside of the South," the study said.
"We now have a clearer picture of who stands to lose financial assistance and join the ranks of the uninsured if the Supreme Court rules in favor of King," said Andy Hyman, senior program officer of the Robert Wood Johnson Foundation, which funded the study.
"The court will decide if people living in some states will continue to receive financial help buying coverage while their neighbors in the next state will not and remain uninsured," Hyman said.
Read MoreObamacare ruling could take insurance from millions
Every state in the South, with the exception of Florida and Virginia, voted for the Republican challenger Romney over the incumbent Obama in 2012. Support for Romney was particularly strong among white residents in that region.
And, "most whites in general, but particularly whites from the South, have unfavorable views toward [Obamacare], and that goes for the middle-income group as well," said Mollyann Brodie, a senior vice president at the Kaiser Family Foundation, citing recent public polling by that health-care policy research organization.
Every state in the South with the exception of Kentucky is served by the federal Obamacare exchange HealthCare.gov, since their governments rejected the option of running their own ACA marketplace. Thirteen states and the District of Columbia are operating their own health insurance marketplaces.
The Supreme Court case hinges on a purported legal distinction between a federally run exchange and one operated by a state. The high court is set to hear arguments in the case on March 4.
The ACA explicitly says that subsidies or tax credits to help pay for premiums as well as for out-of-pocket health costs can be issued to customers of a state-established exchange. Although the ACA talks about the creation of a federal exchange in states that choose not to run their own marketplace, it doesn't say anything explicitly about such subsidies being issued to customers of such an exchange.
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The Obama administration nonetheless claims the HealthCare.gov subsidies are legal because the ACA contemplates the need for HealthCare.gov, and that the ACA's goal of significantly reducing the number of uninsured implicitly means that tax credits have to be available to customers nationally, regardless of the particular flavor of exchange.
The tax credits are worth billions of dollars, and helped to lower premium costs for a large majority of enrollees of HealthCare.gov. Such tax credits issued to customers of state-run exchanges are not at risk from the Supreme Court challenge.
The administration filed a brief Wednesday night with the high court outlining its position in defense of the subsidies.