WASHINGTON — Sen. Susan Collins all but closed the door Sunday to supporting the last-ditch Republican health care bill, leaving her party’s drive to uproot President Barack Obama’s health care law dangling by an increasingly slender thread.

Already two Republican senators, Rand Paul of Kentucky and John McCain of Arizona, have said they would vote against the legislation. All Democrats oppose the measure, so “no” votes from three of the 52 Republican senators would kill the party’s effort to deliver on its perennial promise to repeal “Obamacare.”

“It’s very difficult for me to envision a scenario where I would end up voting for this bill,” said Collins, a Maine moderate.

Susan Collins

Collins’ all-but-certain opposition leaves the White House and party leaders desperate to rescue their promise to repeal Obama’s Affordable Care Act with one immediate option: trying to change the mind of at least one opponent.

Republicans have said they’re still reshaping the bill in hopes of winning over skeptics. Collins said sponsors were making last-minute adjustments in the measure’s formulas used to distribute federal money to the states, and the measure’s sponsors said they still intended to plow ahead.

“So yes, we’re moving forward and we’ll see what happens next week,” said one of the authors, Sen. Lindsey Graham, R-S.C.

Paul criticized the Republican bill anew as “not repeal.” He said he opposed a key pillar of the legislation – transforming much of the federal spending under Obama’s law into block grants of money that states could spend with wide latitude. He said the Republican bill left too much of that spending intact and simply gave states more control over it.

“Block-granting Obamacare doesn’t make it go away,” Paul said.

Collins said she had a lengthy conversation Saturday with Vice President Mike Pence, who she said urged her “to think more thoroughly about some issues.” Graham suggested backing a proposal sought by Paul that would make it easier for people to join or form group insurance plans so they would have lower premiums.

Collins said she was troubled by the bill’s cuts in the Medicaid program for low-income people. She expressed concerns that the measure would result in many people losing health coverage and didn’t like a provision letting states make it easier for insurers to raise premiums on people with pre-existing medical conditions.

As Republican leaders scramble for votes, a chief target is Sen. Lisa Murkowski, R-Alaska, whose state has unusually high health care costs because of its many remote communities. Collins and Murkowski were the only Republicans who voted “no” on four pivotal votes on earlier versions of the Republican legislation this summer.

Murkowski has remained uncommitted on the newest bill, saying she’s studying its impact on Alaska. Her state’s officials released a report Friday citing “unique challenges” and deep cuts the measure would impose on the state.

A showdown vote would have to occur this week to give Republicans any shot at reversing their debacle on the issue in July, when the Republican-run Senate rejected their initial attempt to dismantle Obamacare. When September ends, Republicans will lose procedural protections that have blocked Democrats from successfully stalling the bill; after that, Republicans would need 60 votes to move ahead.