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 said federal deficits would fall by $119bn (£91bn) from 2017-2026 under the proposal, which is championed by President Donald Trump.
House Republicans were criticised for passing the bill before the assessment.
Wednesday’s rating lays down the gauntlet to Republican senators who are now crafting 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 compared with 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 CBO scoring of the healthcare reform plan approved by the House of Representatives landed with a sodden thud in Washington on Wednesday evening. Now Senate Republicans are left with an unpleasant mess to clean up.
If the analysis of the original bill was bad, the changes House leadership implemented to make the legislation palatable to a majority of their chamber have only made things worse.
The House bill hasn’t even been formally handed over to the Senate yet, but when it is it won’t just be dead on arrival, rigor mortis will have set in.
Senate Leader Mitch McConnell will essentially have to start from scratch in coming up with a reform plan that can pass his chamber. Given that different Republicans want different things out of the legislation – some want to preserve Medicaid healthcare for the poor, others want complete repeal of Obamacare – it will be a Herculean task.
The Senate will be pressed to come up with something, however, and success isn’t inconceivable. Then the challenge becomes finding common ground with the House and squeezing the compromise legislation through.
That’s the highest bar of all – one that looks set at the top of a political Mount Everest.
The findings place a big question mark over whether Republicans will be able to muster enough support to send a 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.”
Obamacare has been rocked by insurers leaving the online marketplaces that sell medical insurance, and Blue Cross Blue Shield announced on Wednesday it would be the latest company to withdraw from the exchanges, in Missouri and Kansas.
The insurer said it had lost more than $100m.
The Republican legislation would eliminate most Obamacare taxes that help subsidise private health coverage for individuals.
It would also impose deep cuts on the government’s Medicaid health plan for the poor and disabled.
Many of the 20 million Americans who gained coverage under Obamacare did so through the expansion of Medicaid.
Republicans say the AHCA aims to fix the rising premiums that many Americans complained about under Obamacare, and which have long been a problem in the US healthcare system.
The revised bill would provide $8bn over five years to help sick people, including those with cancer, pay for coverage.
But many health policy experts say the amount is far from enough to cover the cost.