Some 22 million Americans could lose their health insurance over the next decade under a Senate bill to replace Obamacare, a congressional report says.
However, the bill would reduce the budget deficit, the non-partisan Congressional Budgetary Office said.
Similar legislation passed by the House was also said to leave millions uninsured. Some Republicans have voiced reservations about the plan.
But the White House disputed the CBO’s figures.
Responding to Monday’s report, it said: “The CBO has consistently proven it cannot accurately predict how healthcare legislation will impact insurance coverage.”
The report is a review of draft legislation unveiled by the Republican party last week.
It is unlikely to be approved by Democrats, who see the proposals as cruel and unfair.
The CBO said that 15 million more people would be uninsured by 2018 under the proposed legislation than under current law, largely because the penalty for not having insurance would be eliminated.
More pain – Anthony Zurcher, BBC News, Washington
Donald Trump called the House healthcare bill that resulted in 23 million additional uninsured by 2026 “mean”. How might he describe a Senate version that only reduces those numbers to 22 million?
Less affluent Americans currently on government-managed Medicaid insurance, many of whom supported the president, will see a 26% reduction in their programmes funding over the next decade, resulting in fewer covered and skimpier benefits.
That is a tough pill for moderate Republicans, who will be under enormous pressure from their constituents to vote against the bill.
There is some good news in the CBO report, of course. Thanks to the cuts, the Senate measure would reduce the budget deficit by a total of $321bn (£252bn) in 2017-2026 – even with Obama-era tax cuts rolled back. The personal health insurance marketplace is estimated to remain largely stable, and premiums will go down (after a spike next year).
The bottom line, however, is that the CBO report is a blow to the Senate bill’s chances of success. House Republicans weathered a similar beating and rallied to pass their version, so it is much too early to write a political epitaph. That assumes, of course, that their efforts are not undermined by the man in the Oval Office.
President Donald Trump’s Republican party is struggling to secure the 50 votes it needs to get its bill through the Senate when it comes to the floor.
The Republican senator for Maine, Susan Collins, has said she will vote against it, joining colleagues Ted Cruz, Ron Johnson, Mike Lee and Rand Paul who on Thursday said they were “not ready to vote for this bill”, but were “open to negotiation”.
Their concerns about the measure include its rollback of the expansion of Medicaid, the public health programme for the poor and disabled.
Mr Johnson said on Sunday there was “no way” there should be a vote on the legislation this week.
The Senate bill would slash taxes for the wealthy offering less help for working families to buy medical insurance.
Top Democrat Nancy Pelosi earlier warned that “hundreds of thousands” of Americans would die if congressional Republicans pass their healthcare legislation.
Republicans have rejected the notion that anyone will die as a result of their healthcare plan.