WASHINGTON — Senate Republicans’ bill to erase major parts of the Affordable Care Act would cause an estimated 22 million more Americans to be uninsured by the end of the coming decade – only about a million fewer than similar legislation recently passed by the House, the Congressional Budget Office said in a forecast released Monday.

Sen. Susan Collins, R-Maine, reacted to the analysis by Congress’ nonpartisan budget scorekeepers by saying she would vote against advancing the Senate health care bill, casting more doubt on the likelihood that Majority Leader Mitch McConnell will get the legislation through the Senate this week before Congress breaks for the Fourth of July recess.

Collins issued three tweets voicing her opposition to the bill Monday night.

“I want to work with my GOP and Dem colleagues to fix the flaws in ACA (Affordable Care Act). CBO (Congressional Budget Office) analysis shows Senate bill won’t do it. I will vote no on mtp (motion to proceed),” she tweeted.

Her other tweets mention the CBO forecast that 22 million fewer people would have insurance by 2026 under the Senate bill than if the ACA remained in place, and that Medicaid cuts will hurt America’s most vulnerable. Collins also tweeted that the Senate bill “doesn’t fix ACA problems for rural Maine. Our hospitals are already struggling. One in 5 Mainers are on Medicaid.”

The CBO analysis also estimates that the Senate measure, drafted in secret mainly by McConnell and aides, would reduce federal spending by $321 billion by 2026 – compared with $119 billion for the House’s version.

The CBO estimates that two-thirds of the drop in health coverage a decade from now would fall on low-income people who rely on Medicaid. And among the millions now buying private health plans through ACA marketplaces, the biggest losers would roughly parallel the ones under the House’s legislation: The sharpest spike in insurance premiums would fall on middle-aged and somewhat older Americans.

The CBO’s analysis has been awaited as a crucial piece of evidence as McConnell, of Kentucky, and other Republican leaders try to hurry a vote on the bill this week. They have been navigating an expanding minefield of resistance from their party’s moderate and conservative wings, while Democrats, and independents such as Sen. Angus King of Maine, are united against the measure.

The release of the 49-page report late Monday afternoon seemed to worsen the bill’s prospects. No new senators immediately said they would back the legislation, and Collins was among three members of the Republican caucus who said they would vote against starting debate Tuesday on the bill in its current form. A fourth, Sen. Dean Heller, R-Nevada, had expressed his opposition last week.

I want to work w/ my GOP & Dem colleagues to fix the flaws in ACA. CBO analysis shows Senate bill won’t do it. I will vote no on mtp. 1/3

— Sen. Susan Collins (@SenatorCollins) June 26, 2017

The Senate bill could undergo further changes before such a vote is scheduled, which could prompt these senators to reconsider. If the Democrats vote as a bloc, McConnell can afford to lose no more two Republican votes for a procedural motion to succeed.

Asked whether McConnell and other Senate leaders had amassed enough support to pass the measure, Sen. Roy Blunt, R-Missouri, responded, “Anyone would tell you they don’t.”

Conservative Sen. Rand Paul, R-Ky., labeled the Better Care Reconciliation Act “a terrible bill” and repeated his contention that it would not go far enough in repealing the sprawling health-care law enacted seven years ago by a Democratic Congress and president.

McConnell took to the Senate floor just before the report’s release to press anew for rapid action. He made it clear that the bill, already tweaked early Monday, could be negotiated further to try to win over holdouts.

“The American people need better care right now,” McConnell said. “This legislation includes the necessary tools to provide it.”

Democrats quickly seized on the CBO’s projection of how much the ranks of the uninsured would grow. “No matter how the bill changes around the edges, it is fundamentally rotten at the center,” said Senate Minority Leader Charles Schumer of New York.

Former CBO director Doug Holtz-Eakin, a Republican who is now president of the American Action Forum, said the report draws basically the same conclusions as the budget office’s earlier analysis of the House measure. He predicted that Republican senators are “going to get beaten on the head with the CBO report like it’s a club.”

The fresh figures come as President Trump, in a sharp pivot from the praise he initially lavished on the House bill, has been urging the Senate to provide Americans more generous help with health insurance. On Sunday, the president repeated during a “Fox and Friends” TV appearance a word he had used in a private White House lunch with a group of GOP senators earlier this month: that the House’s version is “mean.”

According to the CBO, the Senate bill would mean that an estimated 15 million fewer Americans would have coverage next year, compared with the number if the ACA remained in place. At the end of the decade, the 22 million increase in the ranks of the uninsured would include 15 million low-income Americans who would otherwise be on Medicaid and 7 million with private insurance.

The Senate plan would reduce federal spending to help people afford premiums for individual health insurance policies significantly more than under the House version. The Senate’s version would cut spending on tax credits by $408 billion by 2026 – compared with a $276 billion reduction in the House plan. The difference lies, in part, because the Senate’s version would not permit people with incomes as high to qualify for tax credits and would restrict federal help to health plans sold through the ACA’s marketplaces. The Senate would tie the tax credits to skimpier health plans than the current subsidies.

And while the Senate bill would phase out the ACA’s Medicaid expansion more slowly than the House legislation, cuts to the public insurance program for the poor still would account for by far the largest share of the reduction in federal spending under the Senate bill – $772 billion over the coming decade. In a briefing for reporters, CBO staff members said that they had not analyzed the bill’s effects on Medicaid cuts beyond the coming decade, but that the reductions inevitably would be greater for a second decade.

While they differ in important details, both the Senate Republicans’ plan and the American Health Care Act narrowly passed by House Republicans in May share the goal of undoing central aspects of the ACA.

Both bills would eliminate enforcement of the ACA’s mandate that most Americans carry health insurance, relying on subtler deterrents to keep people from dropping coverage. The House version would let insurers temporarily charge higher rates to those who let their coverage lapse, while the Senate added a provision Monday would let health plans freeze out customers for six months if they let it lapse.

In different ways, both would replace federal subsidies that help the vast majority of consumers buying coverage through ACA marketplaces, instead creating smaller tax credits that would provide greater assistance to younger adults while making insurance more expensive for people from middle age into their 60s.

After two years, both also would end subsidies that now help about 7 million lower-income people with ACA health plans afford deductibles and copays. And both would repeal an array of taxes that have helped to pay for the ACA’s benefits, including levies on health insurers and on wealthy Americans’ investment income.

For the Senate bill, the CBO’s estimates of insurance coverage and federal spending are influenced by the fact that its forecast covers a 10-year window and the legislation’s most profound changes for the nation’s health-care system are tilted toward the latter part of that period.

The bill would, for instance, leave in place the ACA’s expansion of Medicaid through 2020. After that, it would begin a three-year phaseout of the federal money that under the ACA has paid almost the entire cost of adding 11 million Americans to the program’s rolls in 31 states.

That means the extra funding wouldn’t disappear until the mid-2020s – roughly when sharp new restrictions on federal payments for the entire Medicaid program would take effect.

Over the weekend, the senior Democrat on the Senate subcommittee that oversees the CBO said in a tweet that he had asked the budget office to estimate the Senate bill’s effect on insurance coverage over a longer time horizon. “GOP is hiding the worst Medicaid cuts in years 11, 12, 13 and hoping CBO stays quiet,” Sen. Chris Murphy, D-Conn., wrote.

Portland Press Herald Staff Writer Dennis Hoey contributed to this report.