Republicans close to Senate Majority Leader Mitch McConnellMitch McConnellOvernight Finance: CBO: 22M more uninsured with Senate ObamaCare bill | Trump gets green light for partial travel ban | GOP: ObamaCare taxes must go Senate Dems plan floor protest ahead of ObamaCare repeal vote Senate Dems set principles for potential budget negotiation MORE (R-Ky.) are confident he can save the Senates teetering healthcare reform bill, the Better Care Reconciliation Act, over the next five days though it's shaping up to be a daunting task.

Some Republican senators are skeptical that McConnell can stick to his ambitious plan of passing a major overhaul of the nations healthcare system before Congress takes off for the July 4 recess.

But the majority leader and his closest allies believe he can wear down rebels in his conference after 20 hours of floor debate and an unlimited amendment process, which will give colleagues a chance to hammer out a compromise.

A GOP aide familiar with internal discussions said Senate Republicans are going full steam ahead in anticipation of a vote later in the week.

I have no indication that anything is off in terms of the floor. Were preparing to go in every possible way, the aide said Sunday.

Some GOP lawmakers have their doubts, however.

Sen. Susan CollinsSusan CollinsIs Senate ObamaCare repeal bill too mean? Funeral for the filibuster: GOP will likely lay Senate tool to rest This week: Senate races toward ObamaCare repeal vote MORE (R-Maine), one of the chambers leading moderates, said the bill needs to be reworked.

Its hard for me to see the bill passing this week, she said Sunday on ABCs This Week.

Five Republican senators four conservatives and moderate Dean HellerDean HellerIs Senate ObamaCare repeal bill too mean? Cornyn: GOP won't delay ObamaCare repeal vote This week: Senate races toward ObamaCare repeal vote MORE (Nev.), who is facing a tough reelection race in 2018 have announced their opposition, and several other centrists are voicing serious concerns.

Republicans control 52 seats and can afford only two defections to still pass the bill, assuming no Democrats vote for the measure. Vice President Pence may need to be called in to break a 50-50 tie.

But the math is complicated; making changes that conservatives demand would cause more moderates to come out against the bill, while appeasing centrists by watering down cuts to Medicaid risks a rebellion from the GOPs right flank.

Even so, McConnells inner circle believes he can put together 50 votes in the next week to pass the legislation because of his most powerful argument: failure is not an option.

Senate Republican Policy Committee Chairman John BarrassoJohn BarrassoA bipartisan consensus against 'big pharma' is growing in Congress McConnell allies confident in healthcare win Dem senator: GOP's healthcare approach will 'devastate Medicaid' MORE (Wyo.), a member of McConnells leadership team,on Sundaysaid that leaders have enough flexibility within the legislation to address the concerns of colleagues who last week said they cant support it.

I believe we do. I believe well get it passed, he said on "Fox NewsSunday."

Thats the only way we can fundamentally change away from ObamaCare, get rid of all the hated mandates and the taxes and put Medicaid on a sustainable course long term.

Josh Holmes, McConnells former chief of staff, says the leader will find a way to balance the concerns of GOP critics on both sides of the ideological spectrum.

I think theyre having conversations over the weekend that hopefully will send that in the right direction, he said of the legislation.

While conservatives such as Sens. Ted CruzTed CruzTrump phones Senate holdouts on GOP healthcare bill A bipartisan consensus against 'big pharma' is growing in Congress Cornyn: GOP won't delay ObamaCare repeal vote MORE (R-Texas) and Mike LeeMike LeeTrump phones Senate holdouts on GOP healthcare bill A bipartisan consensus against 'big pharma' is growing in Congress Cornyn: GOP won't delay ObamaCare repeal vote MORE (R-Utah) have criticized the bill for leaving some of ObamaCare's regulatory reforms intact and Sen. Ron JohnsonRon JohnsonTrump phones Senate holdouts on GOP healthcare bill The Hill's 12:30 Report Cornyn: GOP won't delay ObamaCare repeal vote MORE (R-Wis.) says he needs more time to decide, they have left themselves room to vote yes, Holmes said.

They've had every every opportunity to draw a hard line and and say no. And they're leaving themselves some some wiggle room, he said.

McConnell can argue to conservatives that voters expect action after Republican candidates spent the past seven years campaigning against ObamaCare. The alternative to passing the Senate bill is to live with the current law.

McConnell can tell nervous moderates that the Medicaid expansion under ObamaCare is not sustainable and that reforming the program is not necessarily a political liability.

Its important to point out that this bill has yet to undergo any debate, amendments, negotiated changes. The White House has yet to engage; were at the starting line here, said Scott Jennings, a former senior political adviser to McConnell. Were a long way to say that people are definitely going to vote against it.

Jennings said McConnell, as a representative of a state that expanded Medicaid enrollment, is well positioned to persuade moderates to accept the bill.

There is absolutely no electoral empirical evidence whatsoever that favoring repeal of ObamaCare or favoring Medicaid reform or even repeal of the expansion has ever cost a Republican an election, he said. We just won the White House and the entire Congress and just about the entire party ran on repealing ObamaCare.

Senate GOP leaders expect an analysis from the Congressional Budget Office on Monday. That report will give them a better idea of how many Republican votes are up in the air.

A vote on a motion to proceed to the bill, which requires only a simple majority to pass under the budget reconciliation process, could happen as soon asTuesday.

Also this week, Senate Republican and Democratic aides will present their competing arguments to the Senate parliamentarian for why key sections of the bill either comply with or run afoul of the chambers budget reconciliation rules, a process known as the Byrd bath.

One Senate aide familiar with the process says the Byrd bath named after late Sen. Robert Byrd (D-W.Va.), who established the six-part test for determining whether legislation is eligible to pass with a simple majority takes about a day.

McConnell has his work cut out for him.

He must persuade at least three of five GOP colleagues who oppose the Senates healthcare bill to flip their positions over the next five days.

One of them, Sen. Rand PaulRand PaulTrump phones Senate holdouts on GOP healthcare bill Cornyn: GOP won't delay ObamaCare repeal vote GOP senator: Leadership trying to 'jam' through healthcare bill MORE (R-Ky.), his home state colleague, is viewed as irretrievably gone. He has panned the core of the bill a system of tax credits to help low-income people buy health insurance as "ObamaCare Lite."

Another, Cruz, has had a notoriously tense past relationship with McConnell and at one point called him a liar on the Senate floor.

A third, Heller, is the Senates most vulnerable incumbent andon Fridayslammed the bill in such harsh terms that it will be very tough for him to change his mind without a major rewrite of the legislation, lest he be accused of a flip-flop.

John Weaver, a Republican strategist who served as a chief presidential campaign strategist to Ohio Gov. John Kasich (R) who opposes the Senate bill predicts that McConnell will come up with some concessions for Cruz, Lee and Johnson to spur them to vote yes.

He said they will be accommodated with some fig leafs, not dissimilar to what happened in the House with some of the Freedom Caucus members.

He said yes votes could be tougher to secure from moderates, but McConnell may be able to corral them by adding more money to the bill. Right now it includes only an extra $2 billion to fight opioid addiction, far less than the $45 billion that Sen. Rob PortmanRob PortmanMcConnell allies confident in healthcare win GOPs message on ObamaCare is us versus them Sanders: GOP healthcare bill is a 'moral outrage' MORE (R-Ohio), a potential swing vote, wanted.

I do worry that McConnell will agree to add to x-billions of dollars and it will be enough for a couple of these moderates to say, 'I held out and they added some more cash and now I can support the bill, Weaver said.

This report was updated at 8:02 a.m.

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McConnell allies confident in healthcare win - The Hill