That grim prediction by the Urban Institute was laid out as Trump prepares to assume the presidency and as Republicans make plans to repeal Obamacare during the next session of Congress in 2017.
House Speaker Paul Ryan, R-Wisc., said this week that while Congress will seek a full repeal of the ACA immediately, there will be a transition period, of some unspecified duration, that would avoid loss of coverage for the 20 million or so people who had gained health insurance from Obamacare.
But it's not clear how many of those people would retain coverage under a GOP replacement plan for Obamacare.
The Urban Institute's study does not factor in any replacement measures. Instead, after noting that there is "currently no consensus" on a replacement plan, it considers what would happen if Congress through the budget reconciliation process partially repealed the ACA.
The study also assumes that Congress, just as it did with a similar effort that was vetoed earlier this year by President Barack Obama, would delay the budgetary effects for two years, until 2019.
The study's authors noted that approach would allow the elimination of Medicaid expansion, which led to coverage gains among many more poor adults, as well as the ending of federal financial aid for most Obamacare private insurance customers. Also gone under that scenario is the individual mandate, which requires most Americans to have health coverage or pay a fine, and the employer mandate, which requires most large employers to offer affordable coverage to workers or be subject to a fine.
Left in place would be ACA provisions that prohibit insurers from discriminating against sicker customers, and minimum health benefits in insurance plans.
Under that scenario, according to the study, "the number of uninsured people would rise from 28.9 million to 58.7 million in 2019."
"The share of nonelderly people without insurance would increase from 11 percent to 21 percent, a higher rate of uninsurance than before the ACA because of the disruption to the nongroup insurance market," the study said.
Researchers estimated that 82 percent of the people who became newly uninsured would be in working families, and that 56 percent would be non-Hispanic whites. And 80 percent of the adults who became uninsured would not have college degrees, the study found.
The study also noted that unless Congress took action, there would also be "very large increases in demand" for so-called uncompensated care on state and local governments, as well as on health providers like hospitals. Uncompensated care refers to treatment of people who lack health insurance.
On the other hand, the study noted, federal spending on health care for the nonelderly would be reduced by $109 billion in 2019 alone. And federal spending on health care for the nonelderly would be reduced a total of $1.3 trillion from 2019 through 2028 because of the elimination of funds that supported Medicaid expansion, as well as subsidies that lowered the premium and out-of-pocket health costs of Obamacare customers.