Mr Trump told 49 Republican senators at a White House lunch on Wednesday that he wanted more than a straight repeal. If no replacement plan were in place three years after enactment, 27 million more people would be uninsured, with that number jumping to 32 million within 10 years after enactment, including 19 million people cut off from Medicaid.
Premiums would double by 2026 as well.
Senate Majority Leader Mitch McConnell plans to hold a repeal-only vote on health care next week.
The Republicans also received another blow to their health care bill Friday when the American Medical Association, the nation's largest doctors' group, urged the senators to stop their efforts to repeal and replace the ACA and instead begin a bipartisan effort to stabilize the insurance marketplace.
"If Republicans don't Repeal and Replace the disastrous ObamaCare, the repercussions will be far greater than any of them understand!" But the CBO report lays out what the potential long-term effects of repeal without replacement would be.
The GOP-proposed bill would also reduce the federal deficit by 473 billion USA dollars from 2016 through 2026, according to the report.More news: Uber rival Grab to raise $2.5bn, mostly from Didi and SoftBank
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Still, at least two Republican senators Sunday appeared to reaffirm their intention to vote against the procedural motion if it involved the latest version of the GOP's repeal-and-replace bill.
Additionally, without people being required to have insurance, there would be little to no financial incentive for some coverage providers to offer individual policies.
The forecast by Congress's nonpartisan budget scorekeepers is similar, though not identical, to updated estimates from January that they issued of the repeal-only legislation that passed the House and the Senate in late 2015 before being vetoed by then-President Barack Obama.
Even with the new score, CBO did not have time to analyze an amendment added to the Senate bill by Sen. The disabled, women and children and others. Trump said his plan was to "let Obamacare fail", and, if it does, he said he would not "own it".
Senate leaders huddled with members on Wednesday night to iron out differences with the repeal and replacement bill.
BBC reports that new opposition came from two Republican senators, which made it hard for the bill to pass. Senators Mike Lee of Utah and Jerry Moran of Kansas joined other party opponents including Susan Collins of Maine.