The latest Republican bill to repeal the Affordable Care Act (ACA) appears doomed to fail after a key senator came out strongly against it within minutes of an analysis which said the plan would strip health insurance from “millions” of Americans.

Moments after the nonpartisan Congressional Budget Office (CBO) released a partial analysis of the Republicans’ plan, Maine senator Susan Collins announced she would not vote for the bill and called it “deeply flawed”.

The CBO said it was unable to provide its customary detailed estimate because it lacked time to examine the legislation, but said “millions fewer” people would have insurance because of large cuts to Medicaid (a health insurance program for the poor), lower subsidies for individuals who buy insurance, and a lack of penalties for people who prefer to go without insurance.

Collins said the bill would have a “substantially negative impact on the number of people covered by insurance”.

The estimate, which says the bill would cut the US budget deficit by $133bn, comes as Senate Republicans hoped to push health reform through this week. It appears that with a narrow 52-48 Senate majority, they have already lost too many votes. No Democrats plan to vote for the bill, which means at least 50 Republican senators would be needed with the casting vote of Mike Pence, the vice-president.

Collins joined Rand Paul of Kentucky and John McCain of Arizona in pledging to vote against the bill, leaving only 49 votes. Late on Monday, Donald Trump stepped back into the fray with a renewed attack on McCain, in a sarcastic tweet that contained clips of previous comments in which the senator promised to repeal and replace the ACA, which is popularly known as Obamacare. “My oh my has he changed – complete turn from years of talk!” Trump said.

The president’s vilification of McCain provoked some of the liveliest exchanges at a town hall debate held by CNN on Monday night. Lindsey Graham, a senator for South Carolina, rallied to the side of his close friend, despite McCain’s opposition to the proposals he co-authored.

Graham told CNN’s audience: “John, if you’re listening … nobody respects you more than I do. So to any American who has a problem with John McCain’s vote, all I can tell you is that John McCain was willing to die for this country and he can vote any way he wants to and it doesn’t matter to me.”

Bernie Sanders described Trump’s latest attack as “beyond my imagination. I cannot understand how someone like Donald Trump could attack an American hero, one of the most decent people in the United States Senate”.

The CBO’s findings, although less specific than usual, still revealed that “millions” of Americans would have reduced health insurance. In particular, Medicaid funding would be significantly shrunk and the Republican bill would shift funds from Democratic states which chose to accept federal funding to insure poor Americans to Republican states which refused to do the same.

The exact number of people could “vary widely” depending on how states implement the legislation, but the CBO said the “direction of the effect is clear”. The biggest effects would begin in 2020.

“In the short time available, rather than provide the point estimates that are typical in such analyses”, the CBO was able to analyze the bill’s effect on “health insurance coverage and market stability”. Analysts said they would need “several weeks” to give a fuller analysis of the bill.

Senate Republicans offered new deals to key lawmakers in the hope of forcing through repeal that would slash the government’s spending on healthcare for poor and disabled Americans.

The Senate must vote on a bill before a Saturday deadline if Republicans are to pass it with a simple majority, but critics, led by McCain, are concerned that the usual due process has been ignored.

“Right now, there is a frenzy of special deals being considered, [and] elbow-twisting behind the scenes to bring swing votes from off the fence,” Senator Ron Wyden, the ranking Democrat on the Senate finance committee, told NPR on Monday. “The fight is far from over.”

Trump and Senate Republicans are trying to meet a seven-year GOP campaign pledge and free up cash for future tax reform. In order to do that without a single Democratic vote, Republicans want to pass the bill using a process called “reconciliation” that allows Republicans to pass bills that affect taxes and spending using just 51 votes. But the reconciliation process is time-bound, tied to a budget resolution Congress passed earlier this year.

Lisa Murkowski of Alaska has not revealed her position. Nevertheless, pressure from the Alaska governor, Bill Walker, to vote “no” has been steadfast. Walker said any change to Medicaid would “likely result in drastic cuts” in his state.

At a hearing in the Senate, Orrin Hatch of Utah made his opening statement over the screams of disabled protesters, whom he asked police to remove. Protesters yelled: “No cuts to Medicaid! Save our liberty!” Hatch said: “If the hearing if going to devolve into a side-show or a forum simply for putting partisan points on the board, there’s no reason for us to be here.”

White House officials said they were optimistic about the bill’s chances, though one senior Republican official told the Guardian passing the bill was “nearly impossible”.

Sarah Huckabee Sanders, the White House press secretary, told reporters: “We know we can’t live with the Obamacare status quo. It’s a complete disaster.”

The White House had undertaken a “repeal by 1,000 cuts”, Wyden said, referring to a number of administration actions that make it more difficult for Americans to sign up for health insurance if they don’t get it from their employers.

This week, the administration announced that websites for people to buy health insurance through the ACA would not operate on weekends. The administration has also cut funding for advertising ACA exchanges, shortened the time Americans have to sign up, and cut the budget for people who help Americans sign up, a complex and time-consuming process.

Over the weekend, Republicans rebalanced proposed federal funding to states, including states that are home to senators who could act as swing votes. According to Politico, Alaska, Arizona and Kentucky would all receive more federal funds.

The bill and the process have proved wildly unpopular with the American public. Just 20% of Americans support the measure, according to a CBS poll released Monday. Most Republicans do not support the bill.

Sabrina Siddiqui contributed to this report