In a major blow to the latest Republican healthcare effort, Sen Susan Collins said on Monday she could not support her colleagues’ measure.

Republicans seeking once again to fulfil their promise of repealing a federal healthcare law could afford few defections, and the loss of Ms Collins - a centrist from Maine - likely deprived them of the needed vote margin to move their bill.

If the loss of Ms Collins does doom the initiative, the collapse will be the latest failure in a string of abortive Republican efforts to translate a perpetual campaign promise - the repeal of Obamacare - into legislation. The party has failed to do so despite controlling the presidency and both houses of Congress. Ms Collins' “no” vote also helped stymie a high-profile push in July.

Hoping to win enough votes to advance the measure, Republicans had sought to lure holdouts like Ms Collins and Alaska Sen Lisa Murkowski. Leadership was scrambling to cobble together enough votes after other members of the caucus - including Senators John McCain, Ted Cruz and Rand Paul - declined to support the measure.

In a statement explaining her decision, Ms Collins cited “sweeping changes and cuts to the Medicaid program”, which provides health insurance to low-income Americans; provisions that would allow states to set rules potentially leading insurers to charge higher premiums to people with pre-existing conditions; and a wall of resistance from health industry representatives.

An effort to ease Maine’s loss of funding would not go far enough, Ms Collins said, because “huge Medicaid cuts down the road more than offset any short-term influx of money”, and she faulted the political calculus behind trying to win her vote.

Faces of Obamacare: The health scheme at the centre of the shutdown Show all 3 1 /3 Faces of Obamacare: The health scheme at the centre of the shutdown Faces of Obamacare: The health scheme at the centre of the shutdown obama.jpg Martin Wolske, 49 and his family, Illinois. His son Eric, 23 (bottom right, dark hair) was recently badly injured in a motorcycle accident. Faces of Obamacare: The health scheme at the centre of the shutdown obama2.jpg Tracy Russo, 31, Washington DC Faces of Obamacare: The health scheme at the centre of the shutdown obama3.jpg Kevin McCollum, 41, and his wife Melissa, 40, Texas

“If Senators can adjust a funding formula over a weekend to help a single state”, she said, “they could just as easily adjust that formula in the future to hurt that state”.

Critics of the Republican repeal effort have complained of a rushed and opaque process that hasn’t left enough time to analyse proposals. On the same day that Ms Collins said she could not support the latest bill, the Congressional Budget Office released a partial analysis that offered ammunition to detractors.

The legislation would slash federal subsidies to help individuals buy health insurance, allocating funding via new block grants to states, and would trim the budget deficit by at least $133 billion, the nonpartisan analyst said.

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But millions fewer people would have health insurance covering “high-cost medical events”, the analysis concludes. The loss in coverage would be driven by “substantially lower” Medicare enrollment, fewer people buying coverage because of a loss in federal subsidies, and a lack of penalties for not having insurance.