Republican Sens. Shelley Moore CapitoShelley Wellons Moore CapitoLawmakers propose boosting park funding with oil money Lawmakers trade barbs, torch Trump at DC soiree Overnight Health Care: Senators unveil bipartisan opioid bill | DOJ to seek reimbursements from opioid companies | Groups looking to end AIDS fear losing ground under Trump MORE (W.Va.) and Susan CollinsSusan Margaret CollinsOvernight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate Winners and losers from the .3T omnibus Senators introduced revised version of election cyber bill MORE (Maine) said on Tuesday that they will not support moving forward with the plan to repeal ObamaCare with a delayed replacement.

"My position on this issue is driven by its impact on West Virginians. With that in mind, I cannot vote to repeal ObamaCare without a replacement plan that addresses my concerns and the needs of West Virginians," Capito said in a statement.

Collins said that she is still a "no" on proceeding to the House-passed bill, which would be used as a vehicle for any Senate action.

“We can’t just hope that we will pass a replacement within the next two years. Repealing without a replacement would create great uncertainty for individuals who rely on the [Affordable Care Act] and cause further turmoil in the insurance markets," she said.

She added that she's recommended Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate Overnight Regulation: Omnibus includes deal on tip-pooling rule | Groups sue over rules for organic livestock | AT&T, DOJ make opening arguments in merger trial Warren presses Mulvaney, Azar on tip pooling MORE (R-Tenn.), the chairman of the Senate's Heath, Education, Labor and Pensions Committee, "begin to hold hearings to examine ways to fix the many flaws in the ACA so that it will work better for all Americans.”

The announcements effectively put the healthcare push in limbo. With a 52-seat majority, Senate Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellYou just can't keep good health policy down Trump threatens to veto omnibus over lack of wall funding, DACA fix Democrats desperate for a win hail spending bill MORE (R-Ky.) can only afford to lose two senators and still let Vice President Pence break a tie.

Sen. John McCainJohn Sidney McCainMcCain asks Trump's CIA pick to explain ties to torture Petraeus: Haspel will explain actions in nomination hearing Afghanistan is our longest war ever and Congress has abandoned all responsibility MORE (R-Ariz.) is in Arizona this week recovering from surgery, so McConnell will not have 50 votes until he returns.

If another GOP senator comes out against moving to the bill, McCain's presence will not save it.

Sen. Rob PortmanRobert (Rob) Jones PortmanLawmakers introduce bipartisan bill to speed up infrastructure permitting Overnight Tech: Zuckerberg breaks silence on Cambridge Analytica controversy | Senate passes sex trafficking bill | EU pushes new tax on tech | YouTube toughens rules on gun videos Senate passes controversial online sex trafficking bill MORE (R-Ohio) said early Tuesday he would need to review the bill, but told reporters that a repeal-only bill "will add to more uncertainty." 

Other senators who balked at the last version of the healthcare bill say they will support moving to the repeal-only bill.

Sen. Jerry MoranGerald (Jerry) MoranOvernight Cybersecurity: House Intel votes to release Russia report | House lawmakers demand Zuckerberg testify | Senators unveil updated election cyber bill Five takeaways from Mark Zuckerberg's media blitz House panel calls for Zuckerberg to testify MORE (R-Kan.), who announced his opposition to the repeal-and-replace bill Monday night, said he would support taking up the House-passed legislation.

"I support the President’s efforts to repeal ObamaCare, and I will vote in favor of the motion to proceed. This should be followed by an open legislative process to craft healthcare policy that will provide greater personal choice, protections for pre-existing conditions, increased access and lower overall costs for Kansans," Moran said.

McConnell is doubling down on the new strategy after he announced on Monday night that the Senate would try to separate ObamaCare repeal and replacement amid growing pressure from President Trump and conservatives.

"I believe we must continue to push forward now. I regret that the effort to repeal and immediately replace the failures of ObamaCare will not be successful. That doesn't mean we should give up," he said from the Senate floor on Tuesday.

The 2015 ObamaCare repeal bill passed the Senate 52-47, with Collins and then-Sen. Mark KirkMark Steven KirkHigh stakes as Trump heads to Hill Five things to watch for at Trump-Senate GOP meeting Giffords, Scalise highlight party differences on guns MORE (Ill.) the only Republicans to oppose it.

Capito, who previously voted for the 2015 repeal bill, added on Tuesday that she has concerns about what would happen to her state's Medicaid expansion and combating opioid addiction.

"All of the Senate healthcare discussion drafts have failed to address these concerns adequately," the West Virginia senator said.

The 2015 measure guts ObamaCare by repealing authority for the federal government to run healthcare exchanges and scrapping subsidies to help people afford plans bought through those exchanges. It zeros out the penalties on individuals who do not buy insurance and employers who do not offer health insurance.

Crucially for moderate senators, the bill would end ObamaCare's Medicaid expansion in two years.

The Senate is expected to be in session until August 11, when members will leave town until early September.

Conservatives and Trump are pressuring GOP senators to back the legislation, noting most of them supported the measure in 2015 when they knew then-President Obama would veto it.

"I don’t see how any Republican senator who voted just 18 months ago for this very piece of legislation could now flip-flop 18 months on with ObamaCare still inflicting so much harm on Americans, and the fact that we campaigned on this for four straight elections," Sen. Tom CottonThomas (Tom) Bryant CottonOvernight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate Overnight Regulation: Omnibus includes deal on tip-pooling rule | Groups sue over rules for organic livestock | AT&T, DOJ make opening arguments in merger trial GOP senators push tougher sentencing for synthetic opioid MORE (R-Ark.) told conservative radio host Hugh Hewitt on Tuesday.

But several GOP senators voiced concerns earlier this year about repealing ObamaCare without a replacement plan hashed out.

Portman, Collins, and GOP Sens. Bob CorkerRobert (Bob) Phillips CorkerTrump signs massive spending bill, backing away from veto threat The Hill's 12:30 Report Deficit hawks encourage Trump veto of spending bill MORE (Tenn.), Bill Cassidy (La.) and Lisa MurkowskiLisa Ann MurkowskiMichael Steele: Congress must lead on cannabis reform and stand with the American public Proposed budget for Indian Health Services won't treat Native American patients equally Keep anti-environment riders for Alaska out of spending bill MORE (Alaska) urged leadership for more time to come up with a repeal plan earlier this year.

GOP leadership initially wanted to pass a repeal-only bill but backtracked over a lack of support. GOP Sen. John ThuneJohn Randolph ThuneSenate GOP chairman calls on Zuckerberg to testify GOP pushes to change Senate rules for Trump With shutdown nearing, focus turns to Rand Paul MORE (R-S.D.) acknowledged on Tuesday that they could still fall short.

"We’ll now have to go through the exercise of sort of re-whipping this, because we are using live ammunition. You know, when we had this 18 months ago in 2015, we had a Democrat president that everybody knew would veto the bill," he told Hugh Hewitt.

Repealing portions of ObamaCare without enacting a replacement could leave 18 million people without health insurance the following year, according to a report released Congressional Budget Office (CBO) in January.

After the elimination of ObamaCare's Medicaid expansion and insurance subsidies, 27 million people would lose insurance, and then 32 million by 2026, the CBO found.

- This story was updated at 11:47 a.m.