The new budget office report, issued after a weekend of protests against repeal, will only add to the headaches that President-elect Donald J. Trump and congressional Republicans face in their rush to take apart Mr. Obama’s health law as they try to replace it with a health insurance law more to their liking.
Republicans cautioned that the report painted only part of the picture — the impact of a fast repeal without a Republican replacement. Senator Orrin G. Hatch, Republican of Utah and the chairman of the Finance Committee, said the numbers represented “a one-sided hypothetical scenario.”
“Today’s report shows only part of the equation — a repeal of Obamacare without any transitional policies or reforms to address costs and empower patients,” he said. “Republicans support repealing Obamacare and implementing step-by-step reforms so that Americans have access to affordable health care.”
Congress last week approved a budget that clears the way for speedy action to repeal the health care law. The votes were 51 to 48 in the Senate and 227 to 198 in the House.
But Republicans have yet to agree on a replacement bill, and existing Republican plans, like one drafted by Representative Tom Price of Georgia, who was selected as Mr. Trump’s secretary of health and human services, have yet to be scrutinized by the budget office. The office provides Congress with the official projections of legislative costs and impact that lawmakers use to formulate policy.
“No wonder President-elect Trump realizes that repeal without replace is the real disaster,” said Senator Chuck Schumer of New York, the Democratic leader. “No wonder he has admonished the Congress not to do plain repeal.”
Republicans now have two powerful reasons to “repeal and replace” together: They hope to protect about 20 million Americans who have gained coverage under the law. And they want a politically acceptable judgment from the Congressional Budget Office on the effects of their alternative.
Mr. Trump’s statement last week that a replacement plan should go hand in hand with repeal efforts had already ignited a sense of urgency among Republicans on Capitol Hill. Over the weekend Mr. Trump said he was close to completing a plan to replace the Affordable Care Act with the goal of “insurance for everybody,” but congressional aides said Tuesday that they had not seen an actual proposal.
Republican congressional leaders are trying to put together a plan that could pass muster with the Trump team and also win approval in the Senate under fast-track procedures that would neutralize the threat of a Democratic filibuster.
House Speaker Paul D. Ryan and Senator Mitch McConnell of Kentucky, the Republican leader, met last week with Mr. Price to hash out alternatives, and they have been in close contact with the relevant committee leaders and staff members to begin hammering out ideas that could come into relief at the end of the month, when Republicans have their annual policy retreat.
Stephen Miller, a former Senate press aide and the incoming senior White House adviser for policy, who has been particularly aggressive in presenting himself as the voice of Mr. Trump on all policy matters, has pushed the notion that a plan should move quickly and in tandem with a replacement measure, rather than in a series of smaller bills, congressional aides said.
The repeal legislation analyzed by the budget office would have eliminated tax penalties for people who go without insurance. It would also have eliminated spending for the expansion of Medicaid and subsidies that help lower-income people buy private insurance. But the bill preserved requirements for insurers to provide coverage, at standard rates, to any applicant, regardless of pre-existing medical conditions.
“Eliminating the mandate penalties and the subsidies while retaining the market reforms would destabilize the nongroup market, and the effect would worsen over time,” the budget office said.
The office said the estimated increase of 32 million people without coverage by 2026 resulted from three changes: About 23 million fewer people would have coverage in the individual insurance market. Roughly 19 million fewer people would have Medicaid coverage. And there would be an increase in the number of people with employment-based insurance that would partially offset those losses.
The estimates by the budget office are generally consistent with projections by the Obama administration and by insurance companies.
In its report, the budget office said that repealing selected parts of the health care law — as specified in the earlier Republican bill — would have adverse effects on insurance markets.
In the first full year after the enactment of such a bill, the office said, premiums would be 20 percent to 25 percent higher than under current law.
Repealing the penalties that enforce the “individual mandate” would “both reduce the number of people purchasing health insurance and change the mix of people with insurance,” as younger and healthier people with low health costs would be more likely to go without insurance, the budget office said.
The Republican bill would have eliminated the expansion of Medicaid eligibility and the subsidies for insurance purchased through Affordable Care Act marketplaces, after a transition period of about two years.
Those changes could have immediately increased the number of uninsured by 27 million, a number that would gradually increase to 32 million in 2026, the budget office said.
Without subsidies, the budget office said, enrollment in health plans would shrink, and the people who remained in the individual insurance market would be sicker, with higher average health costs. These trends, it said, would accelerate the exodus of insurers from the individual market and from the public marketplaces.
As a result, it said, about half of the nation’s population would be living in areas that had no insurer participating in the individual market in the first year after the repeal of marketplace subsidies took effect. And by 2026, it estimated, about three-quarters of the population would be living in such areas.
While writing the Affordable Care Act in 2009 and 2010, lawmakers continually consulted the Congressional Budget Office to understand the possible effects on spending, revenue and insurance coverage. The current director of the budget office, Keith Hall, who signed the report issued on Tuesday, was selected and appointed by Republican leaders of Congress in 2015.
The latest report was requested by Mr. Schumer and two other Democrats, Senators Ron Wyden of Oregon and Patty Murray of Washington.