Republicans have been forced to defend their plan to dismantle the Obamacare healthcare reform after a bipartisan research report showed 14 million Americans would lose medical insurance by next year under the proposal.
The US Congressional Budget Office (CBO) report forecast that by 2026, the number of people without health insurance would increase by 24 million if the House of Representatives' legislation to replace the 2010 Affordable Care Act is adopted.
The findings could make it harder for Republicans to sell the plan - their first major piece of legislation under President Donald Trump - in Congress, especially the Senate.
The Trump administration defended the proposed healthcare overhaul, saying it will offer consumers more choices than Obamacare, former president Barack Obama's signature domestic policy achievement.
Congressional Republicans have vowed for years to undo Obamacare, which expanded health insurance to about 20 million Americans.
But their new effort faces opposition from a range of Republicans - from conservatives who think it does not go far enough to moderates concerned about the impact on coverage and costs.
Doctors, hospitals and other medical providers as well as patient advocates have urged lawmakers to abandon the plan.
White House budget director Mick Mulvaney dismissed the CBO's ability to analyse healthcare coverage and said the focus should not be on how many people were insured. "Coverage is not the end. People don't get better with coverage, they get better with care," he said.
The CBO did point out that federal deficits would fall by $337bn (€320bn) between 2017 and 2026 under the proposed measure.
Mr Trump, who campaigned on a pledge to repeal and replace Obamacare and provide insurance for everybody, has yet to comment on the report.
Democrats say the Republican plan could hurt the elderly, poor and working families while giving tax cuts for the rich. Senate Minority Leader Chuck Schumer said it "is a wreck".
"It's vintage Donald Trump: talks like a populist, but when he acts, it's hard-right, favouring the special interests and hurting the middle class and those trying to get there," the Democrat said.
Health and Human Services Secretary Tom Price said: "Every single American will have access and have the financial feasibility to purchase it."
Overall, the CBO projected that 52 million people would be uninsured by 2026 if the bill becomes law, compared with 28 million who would not have coverage that year if Obamacare remained unchanged.
Separately, a White House analysis showed 26 million people would lose coverage over the next 10 years, website Politico reported, citing an Office of Management and Budget document.
Mr Mulvaney, the budget director, told CNN he was unaware of that document.