Sen. Susan Collins Susan Margaret CollinsMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (R-Maine) signaled on Sunday that she is unlikely to support the latest GOP ObamaCare repeal effort, but said she hasn't made a final decision.

"It’s very difficult for me to envision a scenario where I would end up voting for this bill," she told CNN's "State of the Union."

She added that she has "a number of serious reservations" about the bill, including its impact on "cost and coverage," Medicaid and the "erosion of protections" for pre-existing conditions.

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But Collins stressed that she has not made a final decision and is waiting for the Congressional Budget Office (CBO) analysis that could be released as soon as Monday.

The CBO has warned it will only release a preliminary analysis, and it could be "several weeks" before it can fully determine the impact on the number of Americans who have health insurance or the cost of insurance.

GOP leadership can't afford to lose Collins's vote if they want to pass the ObamaCare repeal bill from GOP Sens. Lindsey Graham Lindsey Olin GrahamMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (S.C.) and Bill Cassidy William (Bill) Morgan CassidyCoushatta tribe begins long road to recovery after Hurricane Laura Senators offer disaster tax relief bill Bottom line MORE (La.), which would replace ObamaCare's insurance subsidies and Medicaid expansion with block grants.

GOP Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (Ariz.) announced on Friday that he couldn't in "good conscience" vote for the legislation.

GOP Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (Ky.) has also said he is opposed to the legislation.

Several other key senators, including Sen. Lisa Murkowski Lisa Ann MurkowskiMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (Alaska), remain on the fence.

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Collins—who was widely viewed as a likely "no" vote after opposing the GOP repeal proposals in July—said on Friday that she was "leaning against" supporting the legislation.

“The premiums would be so high they would be unaffordable,” she told the Portland Press-Herald, referring to the cost for individuals with pre-existing health conditions.

GOP leadership lacks a clear path to getting 50 votes for the Graham-Cassidy bill before the rules allowing Republicans to pass their healthcare with a simple majority—bypassing a Democratic filibuster—expire at the end of the month.

An ObamaCare repeal bill that needs 60 votes likely couldn't pass the Senate, with Democrats unanimously opposed to the effort.

Despite the long odds, Graham, Cassidy and the Trump administration have pledged to keep searching for 50 votes as they try to find a way to pass legislation this week.

Collins said on Sunday that she had a "lengthy" talk with Vice President Mike Pence Michael (Mike) Richard PenceMcConnell locks down key GOP votes in Supreme Court fight Momentum growing among Republicans for Supreme Court vote before Election Day Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election MORE and has been told that they are still making revisions to Graham-Cassidy to try win over enough support.

Trump floated over the weekend that Paul could ultimately be convinced to vote yes, saying that he thinks "he may find a way to get there for the good of the Party!"

Trump also targeted Murkowski, saying in Alaska "deductibles high, people angry!"

Paul opened the door to supporting a significantly revised version of Graham-Cassidy on Sunday, but those changes could threaten support from other factions of the GOP caucus.

"I've always been a yes for repeal," he told NBC. "They could remove the block grants from it, and then we could vote on what we all agree on."

--This report was updated at 10:00 a.m.