And a total of 81 percent of the people who would lose coverage work either full or part time, and 82 percent have modest incomes, but are not poor, the report said.

In a new report issued Thursday that looked for the first time at the demographics of that group, the researchers said that 61 percent of the 6.3 million people who would lose health coverage are white, and 62 percent of them live in the South.

The researchers earlier this month found that 6.3 million people would lose health coverage because they would no longer be eligible for subsidies if the Supreme Court rules that such federal financial aid issued to customers on the HealthCare.gov Obamacare insurance exchange is illegal, as plaintiffs claim. Without those subsidies to help pay for premiums in the 37 states served by the federal exchange, the plans would become unaffordable for that number of people, the Urban Institute said.

Obamacare enrollees who are white, live in the South and have jobs with modest incomes would be disproportionately affected by an adverse ruling for Obama at the high court, which could come this June, the study by Urban Institute researchers found.

The kind of people who were more likely to vote for Mitt Romney over President Barack Obama in the 2012 election also turn out to be the same kind of people who would be most likely to lose their Obamacare health insurance if a looming Supreme Court case goes against the president, according to a new analysis .

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.

Read MoreHealthCare.gov hits 7.1M sign-ups



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.