After falling one vote short this summer, US Republicans have revived efforts to overhaul Barack Obama's landmark health care bill, but skepticism Monday by some in President Donald Trump's party has imperiled the plan.

Momentum for repealing and replacing the Affordable Care Act -- a primary Trump pledge as a candidate -- swelled in the past week, after a group of Republican senators unveiled a bill that would effectively replace Obamacare with block grants to the US states.

Senators returned to Washington with Republicans hoping to ram the bill through in the next 12 days, before a change in procedural rules that currently allow a health care overhaul to pass with a simple 51-vote majority in the 100-member chamber.

But if the bill were to move forward before September 30, it would have to do so without a comprehensive review by the Congressional Budget Office (CBO).

"That's problematic," moderate Republican Senator Susan Collins told reporters.

"I'm concerned about what the effect would be on coverage, on Medicaid spending in my state, on the fundamental changes in Medicaid that would be made without the Senate holding a single hearing to evaluate them."

Collins is one of the three Republicans who voted against the previous Obamacare repeal effort, which dramatically collapsed July 28 when Senator John McCain gave his thumbs down on the plan.

In recent days, the senator from Arizona said he would rely on his state's governor for guidance on whether the new plan, known as the Graham-Cassidy bill, was viable.

As Arizona Governor Doug Ducey endorsed the new plan, its co-author Senator Lindsey Graham insisted the effort was "gaining the momentum we need to repeal and replace Obamacare."

But McCain's hesitation was clear. He expressed frustration with the lack of public hearings or a CBO score of the legislation.

"I am going to continue to look at this as the process goes on," McCain told reporters. "But I want regular order."

The bill's supporters might be eager to avoid a CBO score. In July, the non-partisan body projected that the ranks of the uninsured would grow by 16 million Americans, and premiums would rise 20 percent annually, over the next decade if the previous Obamacare repeal bill became law.

An earlier repeal effort would have led to 32 million fewer insured.

- 'Meaner than ever' -Senator Ron Johnson, one of the bill's sponsors, called Graham-Cassidy a "work in process."

But Democrats insisted the latest bid was worse than previous versions, "a red siren moment" for the nation.

"Trumpcare's back, and it's meaner than ever," Senate Minority Leader Chuck Schumer warned.

"If this bill becomes law, our health care system will be dramatically curtailed, and there will be chaos in many states."

The bill, Schumer noted, allows states to permit insurers to roll back protections for Americans with pre-existing conditions.

Money to states would decline over time, eventually disappearing unless Congress appropriated new funding.

And "the new Trumpcare would plunge a dagger deep into the heart of Medicaid," the government health insurance program for the poor and disabled, by halting its expansion and establishing a per-capita cap on Medicaid spending, Schumer said.

Meanwhile, as Collins and McCain equivocated, conservative Republican Senator Rand Paul was a firm no, largely because Graham-Cassidy maintains nearly all Obamacare taxes and regulations.

"This does not look, smell or even sound like repeal," Paul told reporters, anticipating years of health care marketplace chaos should the bill become law.

"I don't think anybody's realized the enormity of this," he added. "It keeps 90 percent of Obamacare and redistributes the proceeds" to states to use as they see fit.

With Paul and all 48 senators in the Democratic caucus opposed, Republicans could afford just one more no vote.

Beyond McCain and Collins, eyes were on Alaska's Senator Lisa Murkowski, who voted no on the July effort, and Ohio's Rob Portman, who voted for the previous bill but has denounced bids to curtail Medicaid expansion.

Democrats have expressed support for a bipartisan effort aimed at stabilizing Obamacare's insurance exchanges so that millions of Americans could maintain their coverage.