The study comes as presumptive Republican presidential nominee Donald Trump has called for an end to the ACA, as top GOP officials have also demanded. At the same time, Trump has proposed a replacement plan that he says will "broaden health-care access" and "make health care more affordable."
"On day one of the Trump administration, we will ask Congress to immediately deliver a full repeal of Obamacare," Trump's campaign says on the section of its website entitled Healthcare Reform to Make America Great Again.
Trump has suggested several steps to replace Obamacare, including lifting restrictions that inhibit sales of health insurance across states lines, allowing insurance premiums to be fully deductible on tax returns, expanding the use of health savings accounts to bolster savings for medical purposes, block granting Medicaid funds to states and allowing prescription drug importation.
But it is not clear to what extent Trump's proposals would make up for the loss of health insurance by millions of people if Obamacare were repealed.
Buettgens said Trump's replacement plan did not have enough details to analyze how many people would end up having health insurance if Obamacare were repealed.
"Without specific details" such as how much states would get in block Medicaid grants "it's impossible to know how they would impact things," he said.
The Trump campaign did not immediately return a request for comment on the RWJF/Urban Institute study.
Spokesmen for the campaign of presumptive Democratic presidential nominee Hillary Clinton also did not immediately respond to a request for comment on the study. Clinton's own website says she will "Defend the Affordable Care Act and build on it to slow the growth of out-of-pocket costs."
While repealing Obamacare has been an oft-repeated mantra of the GOP since the ACA became law under President Barack Obama, a Democrat, coming up with a viable replacement for it has proved difficult for the party.
That difficulty has grown as the numbers of people covered by Obamacare has increased, raising the risk of a dramatic political backlash for the Republican Party if a repeal-and-replace plan led to a significant reduction in the number of people insured.
According to the Obama administration, about 20 million people have gained coverage so far because of the ACA. And the nation's uninsurance rate is, for the first time ever, below 10 percent.