WASHINGTON — The Senate bill to repeal the Affordable Care Act was edging toward collapse on Monday after the nonpartisan Congressional Budget Office said it would increase the number of people without health insurance by 22 million by 2026.

Two Republicans, Senators Susan Collins of Maine and Rand Paul of Kentucky, said Monday that they would vote against even debating the health care bill, joining Senator Dean Heller of Nevada, who made the same pledge on Friday. Senator Ron Johnson of Wisconsin hinted that he, too, would probably oppose taking up the bill on a procedural vote expected as early as Tuesday, meaning a collapse could be imminent.

“It’s worse to pass a bad bill than pass no bill,” Mr. Paul told reporters.

Ms. Collins wrote on Twitter on Monday evening that she wanted to work with her colleagues from both parties to fix flaws in the Affordable Care Act, but that the budget office’s report showed that the “Senate bill won’t do it.”

The report left Senator Mitch McConnell of Kentucky, the majority leader, with the unenviable choices of changing senators’ stated positions, withdrawing the bill from consideration while he renegotiates, or letting it go down to defeat — a remarkable conclusion to the Republicans’ seven-year push to repeal President Barack Obama’s signature domestic achievement.

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.

The White House discounted the report, saying the budget office had “consistently proven it cannot accurately predict how health care legislation will impact insurance coverage.”

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 “essential benefits,” and that as a result, coverage for maternity care, mental health care, rehabilitation services and certain very expensive drugs “could be at risk.”

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 office’s 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 “mean.”

The budget office had found that under the House bill, the number of people without health insurance would increase by 23 million by 2026 — only slightly more than the 22 million projected for the Senate bill.

“C.B.O.’s report today makes clear that this bill is every bit as mean as the House bill,” Mr. Schumer said. “This 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.”

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 “trying to jam this thing through.” He, too, suggested he would not vote even to begin debating the bill.

“I have a hard time believing I’ll have enough information for me to support a motion to proceed this week,” Mr. Johnson said later on Monday.

Beyond the number of Americans without insurance, the Senate bill’s $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 Senate’s bill: The measure would end the Affordable Care Act’s 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 “working very hard to get there” but were not getting any help from Democrats.

“Not easy! Perhaps just let OCare crash & burn!” Mr. Trump wrote, reiterating his assertion that the Affordable Care Act would be doomed if Congress did not come to its rescue.