Fourteen million Americans would lose coverage next year under plans to reform the US healthcare system, with the number ballooning to 24 million by 2026, Congress has said.
A report by the Congressional Budget Office deals a serious blow to a Republican drive already under fire from both parties and large segments of the medical industry.
It undercuts a central argument President Donald Trump and Republicans have cited for swiftly rolling back the 2010 healthcare overhaul - that the insurance markets created under the statute are "a disaster" and about to implode.
The congressional experts said the market for individual policies "would probably be stable in most areas under either current law or the (Republican) legislation".
The report also flies in the face of Mr Trump's talk of "insurance for everybody," which he stated in January.
He has since embraced a less expansive goal - to "increase access" - advanced by House Speaker Paul Ryan and other Republicans.
Health secretary Tom Price said the report was "simply wrong" and omitted the impact of additional Republican legislation and regulatory changes the Trump administration plans.
In a sign of trouble, Representative Mark Walker, leader of a large group of House conservatives, said the report "does little to alleviate" concerns about the bill including tax credits considered too costly.
The budget office's estimates provide a detailed, credible appraisal of the Republican effort to unravel Barack Obama's 2010 overhaul.
The office has a four-decade history of even-handedness and is currently headed by an appointee recommended by Mr Price when he was a congressman.
Mr Trump has repeatedly attacked the agency's credibility, citing its significant underestimate of the number of people who would buy insurance under "Obamacare".
On the plus side for Republicans, the budget office said the party's plan would reduce federal deficits by 337 billion dollars (£276 billion) over the coming decade.
That is largely because it would cut the Medicaid programme for low-income Americans and eliminate subsidies that Mr Obama's law provides to millions of people who buy coverage.
Administration officials took strong issue with the budget office's projections.
"We believe that our plan will cover more individuals and at a lower cost and give them the choices that they want," Mr Price said.
But leading Democrat Chuck Schumer said the projections show "just how empty the president's promises, that everyone will be covered and costs will go down, have been".
The American Medical Association, which opposes the Republican bill because it would reduce coverage, said the report shows the legislation would cause "unacceptable consequences".
Though many Republicans back the bill, conservatives say it does not go far enough in repealing Mr Obama's law, while moderates whose states expanded Medicaid do not want people losing coverage.
The budget office estimated b y 2026, a total of 52 million people would lack insurance, including 28 million who would have been expected to lack coverage under Mr Obama's statute.
People with lower incomes aged 50 to 64, generally too young for Medicare, would represent a disproportionately large share of the uninsured, and growing numbers of people would lose coverage from jobs.