Bloomberg) -- President Donald Trump and Republican leadersunveiled a nine-page framework to rewrite the nation’s tax code this week to rave reviews from withintheir party.

But now the hard part starts -- with the tax-writing committeesin the House and Senate tasked with settling some of the mostdivisive issues.

Questions from where to set the top income tax rate to whetherthe package should be paid for could easily fracture the GOP.

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With 52 senators in their ranks and little hope of Democraticsupport, Republicans can’t afford to lose more than two members toget a bill passed. They also can’t afford another legislative lossfollowing their failure to repeal the 2010 Affordable Care Act.

“There’s no question that there’s certainly comfort in margins,and we don’t have margins for error. And so each individual senatoris very empowered when it comes to a big issue like this,” No. 3GOP Senator John Thune of South Dakota said, referring to the taxoverhaul. “As we’ve seen now a couple different times, it’s veryeasy to take a big bill like this down.”

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The fate of Trump’s promise of a historic tax revamp could bedetermined by six key Republican senators: Bob Corker, John McCain,Rand Paul, Pat Toomey, Orrin Hatch and Susan Collins.

Some in the group have already laid out their demands for a taxbill. Here’s a closer look at those senators, whose actions will becrucial to getting a tax bill across the finish line:

Bob Corker

Having announced he’ll retire after the 2018 election, thedeficit hawk is free to chart his path. And the two-term Tennesseesenator is setting down a marker, insisting he won’t support a taxbill that adds to the deficit. That could make meeting Trump’spromise for massive tax cuts difficult.

Corker recently struck a deal with Toomey paving the way forbudget legislation that would allow for huge tax cuts in theory,but Corker has said he wouldn’t allow them to balloon the deficit.“With realistic growth projections, it cannot produce a deficit,”Corker said Wednesday. “There is no way in hell I’m voting for it.”He estimated that some $4 trillion in revenue-raisers must beachieved in order to ease his concerns.

Corker also said he wants to “get down to lower corporate ratesand get rid of all these crazy issues that exist in our tax code,”describing his opposition to raising the deficit as a "hard stand"in order to "make sure we stay fiscally sane."

John McCain

This week, the Arizona Republican outlined the same condition ontax legislation that twice proved pivotal in blocking Obamacarerepeal efforts in the Senate: Regular order that allows forhearings, debate, amendments and bipartisan support.

“We need to do it in a bipartisan fashion," McCain said Tuesdayof a tax overhaul. “I am committed, as I’ve said before, to abipartisan approach, such as we’ve been doing in the Armed ServicesCommittee for the last 53 years.”

That may be difficult to reconcile with Senate Republicanleaders’ plans to use the fast-track procedure on taxes that theytried to use on health care.

Will the so-called maverick be satisfied? Initial reaction amongDemocrats to the tax framework indicates firm opposition, butMcCain was more positive, praising the multiple tax hearings thatthe Finance Committee has held and saying he looks forward toreviewing the proposal.

McCain, 81 and battling brain cancer, has a history of buckinghis party on the issue of taxes. He was among the few SenateRepublicans to vote against President George W. Bush’s two separatetax cuts in 2001 and 2003, although he later came to support makingthem permanent.

Rand Paul

The Kentucky libertarian is never an easy vote to win over -- heproved it during the health-care debate by staunchly opposing theSenate’s last opportunity to undo Obamacare before the Sept. 30deadline, complaining that it didn’t go far enough. And now he’sstaking out a far-reaching position on taxes, too, calling for a“large cut of at least 15 percent for every taxpayer” in an Aug. 30op-ed.

Paul is also opposed to paying for a tax cut, describing thepush for revenue neutrality as "a terrible idea" that simply shiftsaround the tax burden and fails to achieve “real tax cuts.”

He called on his party to reject the principle of revenueneutrality, warning it will “result in those with the bestlobbyists, lawyers and accountants being the winners, while mosteveryone else either gets nothing or largely loses out.”

Senator Ted Cruz of Texas has also said a tax plan shouldinclude big cuts, pushing back on the idea of revenue neutralchanges.

In 2015, ahead of his failed bid for the Republican presidentialnomination, Paul rolled out a “flat tax” plan that would impose a14.5 percent individual rate.

Pat Toomey

The second-term Pennsylvania senator is an outspoken cheerleaderfor tax cuts and has argued against the need to pay for such aplan, saying the overarching focus must be on economic growth andthat a revenue-neutral plan would be "anemic."

He successfully pushed for an agreement on a budget vehicle thatallows the tax cuts to add to the deficit. He’s also argued forchanging the rules to extend the budget window for a temporary taxcut from 10 years to as many as 30 years. "I’d like to stretch thatout as much as possible," he said.

Toomey, along with fellow tax wonk Senator Rob Portman of Ohio,are seen as the Senate Finance Committee’s thought leaders on taxpolicy. One of Toomey’s big priorities: "Expensing capital might bethe most pro-growth element of this exercise," he told reportersWednesday. "That’s really really important to me." Another is tocreate an incentive to bring home the estimated $2.6 trillion incorporate profits sitting overseas and set up a territorial systemwhere U.S. companies aren’t subject to "an extra layer of tax fromoverseas income."

Orrin Hatch

A tax overhaul may be the GOP’s last best chance to secure amajor legislative victory in 2017, and the chairman of thetax-writing Finance Committee views it as a political imperative."Very important," Hatch said in an interview Wednesday. "We allfeel the pressure to deliver on taxes."

Hatch, who was a second-term senator during the last bigtax-code rewrite in 1986, is keenly aware of the issue’scomplexity.

He declined to say if he believes the Senate will secure amajority to eliminate the state and local tax deduction, a majorrevenue-raiser targeted in the Trump-GOP framework.

"I’m not going to talk about specifics," he said. "It’s a verycomplex bill to begin with. And we’re going to have to make somevery tough decisions about what we keep and what we don’t."

Hatch’s panel has a 14-12 split between Republicans andDemocrats, which means he can afford to lose no more than oneRepublican if Democrats decline to play ball. He said he hopes towin Democratic support, but conceded that it "would be unique" inthis political environment. "If they want to play politics with it,it’s another matter."

Susan Collins

Arguably the Senate’s most moderate Republican, thealways-meticulous Collins drove a hard bargain on health care thathelped torpedo the push. While she has said little about theupcoming tax debate, nobody is taking her vote for granted.

The Maine senator has voted for numerous tax cuts in the past,including the Bush tax cuts of 2001 and 2003.

But she has also taken some heterodox positions that could be afactor: In 2015, she was the only GOP senator of 55 who broke ranksand voted against a budget measure calling for repeal of the estatetax, which the Trump-GOP framework seeks to do.

In 2008, she voted for a measure to raise the top tax rate forpeople earning more than $1 million.

For now, Collins isn’t revealing any of her thoughts on theframework.

"I’m going to take the weekend to study it," she said.

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