WASHINGTON — The House Republican plan to replace the Affordable Care Act would increase the number of people without health insurance by 24 million by 2026, while slicing $337 billion off federal budget deficits over that time, the nonpartisan Congressional Budget Office said Monday.

Republicans had been bracing for what was almost certain to be a bleak accounting of the legislation’s projected effects. The American Health Care Act, as Republicans call their bill, was already facing widespread criticism from health care providers, some conservatives and a united Democratic Party.

The much-anticipated judgment by Capitol Hill’s official scorekeeper did not back up President Trump’s promise of providing health care for everyone and was likely to fuel the concerns of moderate Republicans. Next year, it said, the number of uninsured Americans would be 14 million higher than expected under current law.

But it also provided talking points for House Republican leaders who need the support of rebellious conservatives to pass the measure: lower deficits, reduced federal spending and tax cuts.

The Trump administration immediately denounced the budget office’s conclusions. Tom Price, the secretary of health and human services, suggested the report offered an incomplete picture because it did not take into account regulatory steps he intends to take, as well as other legislation that Republicans plan as part of their multistep strategy to repeal and replace the health law.

“We disagree strenuously with the report that was put out,” he said at the White House.

Democrats remained steadfast in their opposition. “The C.B.O. score shows just how empty the president’s promises, that everyone will be covered and costs will go down, have been,” said Senator Chuck Schumer of New York, the Democratic leader. “This should be a looming stop sign for the Republicans’ repeal effort.”

The coverage numbers released Monday will make it only more difficult for Republicans to explain why their legislation would improve the country’s health care system. And that could make the bill’s fate in the more narrowly divided Senate much more tenuous.

In a sign of the concern over the coverage projections, Senator Susan Collins, Republican of Maine, said the budget office’s report was “cause for alarm” and “should prompt the House to slow down and reconsider certain provisions of the bill.”

Average premiums for people buying insurance on their own would be 15 to 20 percent higher in 2018 and 2019 than they would be under current law, the budget office said. But after that, premiums would be lower than projected under current law — around 10 percent lower by 2026, the budget office said.

The number of uninsured would shoot up next year by 14 million, the budget office said. Most of the increase in 2018 would result from people choosing not to buy insurance after tax penalties for those without coverage are repealed, but in later years, the office said, the number of uninsured would rise further because of changes in Medicaid, the health program for low-income people.

“Some states would discontinue their expansion of eligibility” for Medicaid, and federal spending per beneficiary would be capped, the report noted.

For people receiving subsidies under the Affordable Care Act, the report said, tax credits proposed by House Republicans “would generally be less generous.” But the market would not collapse. Other changes in the House bill would “lower average premiums enough to attract a sufficient number of relatively healthy people to stabilize the market,” the budget office said.

The budget office estimated that 52 million people would be uninsured in 2026 under the House Republican bill, compared with 28 million projected under current law.

The report foresees huge changes in Medicaid. By 2026, it said, federal Medicaid spending would be 25 percent…