Some 23 million people would lose health insurance over the next decade under the revised Republican healthcare plan, says a non-partisan agency.
Fourteen million people would be uninsured in 2018 alone, according to the Congressional Budget Office.
The analysis also said federal deficits would fall by $119bn (£91bn) from 2017-2026 under the proposal, which is championed by President Donald Trump.
House Republicans were excoriated for passing the bill before the assessment.
Wednesday’s rating lays down the gauntlet to Republican senators who are now producing their own version of the bill, which Democrats have labelled “Trumpcare”.
House Republicans amended their American Health Care Act (AHCA) after it failed back in March to attract enough support to even win a vote in a chamber their party controls.
The bill would accomplish a long-running conservative goal of repealing and replacing former President Barack Obama’s 2010 Affordable Care Act, known as Obamacare.
The CBO score predicts the revised AHCA would leave 1 million fewer Americans uninsured overall than the last version, which said 24 million would lose coverage.
The $119bn projected to be shaved off the federal deficit is less than the $150bn in savings forecast in the previous version of the bill.
The findings place a big question mark over whether Republicans will be able to muster enough support to send the bill to the president’s desk.
“I don’t know how we get to 50 [votes] at the moment,” Senate leader Mitch McConnell told Reuters news agency on Wednesday. “But that’s the goal.”
The legislation would eliminate most Obamacare taxes that help subsidise private health coverage for individuals.
It would also…