But the budget office put Republicans in an untenable position. It found that next year, 15 million more people would be uninsured compared with current law. Premiums and out-of-pocket expenses could shoot skyward for some low-income people and for people nearing retirement, it said.
The legislation would decrease federal deficits by a total of $321 billion over a decade, the budget office said.
Mr. McConnell, the chief author of the bill, wanted the Senate to approve it before a planned recess for the Fourth of July, but that looks increasingly doubtful. Misgivings in the Republican conference extend beyond just a few of the most moderate and conservative members, and Mr. McConnell can lose only two Republicans.
At least some of Ms. Collinsâs concerns could be shared by Senators Lisa Murkowski of Alaska and Shelley Moore Capito of West Virginia, whose rural states would face effects similar to those in Maine.
âIf you were on the fence, you were looking at this as a political vote, this C.B.O. score didnât help you,â said Senator Lindsey Graham, Republican of South Carolina. âSo I think itâs going to be harder to get to 50, not easier.â
He added, âI donât know, if you delayed it for six weeks, if anything changes.â
Under the bill, the budget office said, subsidies to help people buy health insurance would be âsubstantially smaller than under current law.â And deductibles would, in many cases, be higher. Starting in 2020, the budget office said, premiums and deductibles would be so onerous that âfew low-income people would purchase any plan.â
Moreover, the report said, premiums for older people would be much higher under the Senate bill than under current law. As an example, it said, for a typical 64-year-old with an annual income of $26,500, the net premium in 2026 for a midlevel silver plan, after subsidies, would average $6,500, compared with $1,700 under the Affordable Care Act. And the insurance would cover less of the consumerâs medical costs.
Likewise, the report said, for a 64-year-old with an annual income of $56,800, the premium in 2026 would average $20,500 a year, or three times the amount expected under the Affordable Care Act.
The budget office report was a major setback to Senate Republican leaders, but it was too early to declare the legislation dead, and turmoil in health insurance markets could still induce Congress to take action this year. Many people thought the House repeal bill was dead after Speaker Paul D. Ryan pulled it from the floor on March 24, but a slightly revised version was narrowly approved by the House six weeks later.
Senator John Thune of South Dakota, a member of the Republican leadership, suggested that leaders would press forward with the Senate bill. He said that an argument could be made for delaying it âif you thought you were going to get a better policy,â but that that was not the case.
âThis is the best we can do to try and satisfy all the different perspectives in our conference,â Mr. Thune said, adding that he did not think the politics would improve by waiting. âItâs time to fish or cut bait.â
The White House discounted the report, saying the budget office had âconsistently proven it cannot accurately predict how health care legislation will impact insurance coverage.â
The Trump administration says the Senate Republican bill would not cut Medicaid because spending would still grow from year to year. But the Congressional Budget Office said that the bill would reduce projected Medicaid spending by a total of $772 billion in the coming decade, and that the number of people covered by Medicaid in 2026 would be 15 million lower than under current law.
In 2026, it said, Medicaid spending would be 26 percent lower than under current law, and enrollment of people under 65 would be 16 percent lower. Beyond 2026, Medicaid enrollment would keep declining compared with what would happen under current law.
The Senate bill would make it much easier for states to obtain waivers exempting them from certain federal insurance standards, like those that require insurers to provide a minimum set of health benefits. The budget office said that nearly half of all Americans could be affected by these cutbacks in âessential benefits,â and that as a result, coverage for maternity care, mental health care, rehabilitation services and certain very expensive drugs âcould be at risk.â
Before the budget office released its report, the American Medical Association had announced its opposition to the bill, and the National Governors Association had cautioned the Senate against moving too quickly.
The budget officeâs findings immediately gave fodder to Democrats, who were already assailing the bill as cruel. Senator Chuck Schumer of New York, the Democratic leader, said Senate Republicans had been saying for weeks that their bill would be an improvement over the House bill, which President Trump had described as âmean.â
The budget office had found that under the House bill, the number of people without health insurance would increase by 23 million by 2026 â only slightly more than the 22 million projected for the Senate bill.
âC.B.O.âs report today makes clear that this bill is every bit as mean as the House bill,â Mr. Schumer said. âThis C.B.O. report should be the end of the road for Trumpcare. Republicans would be wise to read it like a giant stop sign, urging them to turn back from this path that would be disastrous for the country, for middle-class Americans and for their party.â
The criticism was not confined to the Democratic caucus. Mr. Johnson, one of five Senate Republicans who said last week that they could not support the bill as drafted, told a radio host that Senate leaders were âtrying to jam this thing through.â He, too, suggested he would not vote even to begin debating the bill.
âI have a hard time believing Iâll have enough information for me to support a motion to proceed this week,â Mr. Johnson said later on Monday.
Beyond the number of Americans without insurance, the Senate billâs $321 billion in deficit reduction is larger than the $119 billion that the budget office found for the bill that passed the House.
Earlier Monday afternoon, Senate Republican leaders altered their bill to penalize people who go without health insurance by requiring them to wait six months before their coverage would begin. Insurers would generally be required to impose the waiting period on people who lacked coverage for more than about two months in the previous year.
The waiting period was meant to address a notable omission in the Senateâs bill: The measure would end the Affordable Care Actâs mandate that most Americans have health insurance, but also require insurers to accept anyone who applied. The proposal is supposed to prevent people from waiting until they get sick to buy a health plan. Insurers need large numbers of healthy people, whose premiums help defray the cost of care for those who are sick.
Under one of the most unpopular provisions of the Affordable Care Act, the government can impose tax penalties on people who go without health coverage. Republicans have denounced this as government coercion.
The repeal bill passed by the House last month has a different kind of incentive. It would impose a 30 percent surcharge on premiums for people who have gone without insurance.
Mr. Trump wrote on Twitter on Monday that Republican senators were âworking very hard to get thereâ but were not getting any help from Democrats.
âNot easy! Perhaps just let OCare crash & burn!â Mr. Trump wrote, reiterating his assertion that the Affordable Care Act would be doomed if Congress did not come to its rescue.