The Senate Republicans’ healthcare bill is facing a growing mound of opposition from members of their own party as leadership pushes for a vote by the end of the week.

At least four Republican senators have suggested or said they would oppose a key procedural vote that would allow the Obamacare repeal and replacement bill to proceed.

The defectors include centrist Susan Collins of Maine, moderate Dean Heller of Nevada, and conservatives Rand Paul of Kentucky and Ron Johnson of Wisconsin. Several senators remain undecided.

With a majority of 52 senators, Senate Republican Leader Mitch McConnell can only afford to lose two ‘yea’ votes on the bill. In the event of a 50-50 tie, Vice President Mike Pence would cast the deciding vote.

No Democrats are expected to support the bill.

During a press conference, House of Representatives Speaker Paul Ryan said, “I would not bet against Mitch McConnell...I have every expectation the Senate will move this bill.”

The House narrowly passed its own healthcare measure last month.

The Senate plan significantly scales back Medicaid – a healthcare programme for the poor – repeals Obamacare’s taxes on the wealthy, and defunds Planned Parenthood for a year.

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Republicans have called Obamacare – officially known as the Affordable Care Act (ACA) – “unsustainable”, citing skyrocketing premiums and the unstable health insurance marketplaces created by the law.

Several Republican senators had been waiting on the results of the nonpartisan Congressional Budget Office’s analysis of the bill before deciding whether to vote for it.

The CBO has projected that the healthcare plan would leave 22 million more people uninsured by 2026 than under Obamacare.

Citing the CBO’s report, Ms Collins tweeted that the plan’s Medicaid cuts would hurt the most vulnerable Americans, and that people’s access to healthcare in rural areas would be threatened.

“Senate bill doesn't fix ACA problems for rural Maine,” Ms Collins wrote. “Our hospitals are already struggling. 1 in 5 Mainers are on Medicaid.”

She added that she wants to work with her Republican and Democratic colleagues to fix the flaws in the ACA.

“CBO analysis shows Senate bill won't do it,” she said, adding that she will vote against a motion to proceed on the measure.

As the bill continues to face mounting resistance, dealmakers on Capitol Hill and at the White House are trying to find ways to convince senators to support the measure.

Mr Pence is hosting several conservatives at his house for dinner, including Mike Lee of Utah, who said last week that he could not support the original draft of the bill.