President Donald Trump and Republican leaders unveiled a nine-page framework to rewrite the nation’s tax code last week to rave reviews from within their party. But now the hard part starts — with the tax-writing committees in the House and Senate tasked with settling some of the most divisive issues. Questions from whether to set a rate above 35 percent for top earners to how to limit corporate interest deductions could easily fracture the GOP. With 52 senators in their ranks and little hope of Democratic support, Republicans can’t afford to lose more than two members to get a bill passed. They also can’t afford another legislative loss following their failure to repeal the 2010 Affordable Care Act. These six Republican senators will play key rolls in negotiating the details and getting a tax bill across the finish line:

Bob Corker

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

Corker recently struck a deal with Pennsylvania Sen. Pat Toomey paving the way for budget 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 last week. “There is no way in hell I’m voting for it.” He estimated that some $4 trillion in revenue-raisers must be achieved.

Corker also said that he wants to “get down to lower corporate rates and 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

Last week, the Arizona Republican outlined the same condition on tax legislation that twice proved pivotal in blocking Obamacare repeal efforts in the Senate: regular order that allows for hearings, debate, amendments and bipartisan support. “We need to do it in a bipartisan fashion,” McCain said Tuesday of a tax overhaul. “I am committed, as I’ve said before, to a bipartisan approach.”

That may be difficult to reconcile with Senate Republican leaders’ plans to use the fast-track procedure on taxes that they tried to use on health care. Initial reaction among Democrats indicates firm opposition, but McCain praised the multiple tax hearings that the Finance Committee has held.

McCain, 81 and battling brain cancer, has a history of bucking his party on the issue of taxes. He was among the few Senate Republicans to vote against President George W. Bush’s tax cuts in 2001 and 2003.

Rand Paul

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

Paul is also opposed to paying for a tax cut, describing the push for revenue neutrality as “a terrible idea” that simply shifts around the tax burden and fails to achieve “real tax cuts.” He called on his party to reject the principle of revenue neutrality, warning it will “result in those with the best lobbyists, lawyers and accountants being the winners, while most everyone else either gets nothing or largely loses out.” Sen. Ted Cruz of Texas has also said a tax plan should include big cuts, pushing back on the idea of revenue neutral changes.

Pat Toomey

The second-term Pennsylvania senator is an outspoken cheerleader for tax cuts and has argued against the need to pay for such a plan, saying that the overarching focus must be on economic growth and that a revenue-neutral plan would be “anemic.” He successfully pushed for an agreement on a budget vehicle that allows the tax cuts to add to the deficit. He also has argued for changing the rules to extend the budget window for a temporary tax cut from 10 years to as many as 30 years. “I’d like to stretch that out as much as possible,” he said.

Toomey, along with fellow tax wonk Sen. Rob Portman of Ohio, are seen as the Senate Finance Committee’s thought leaders on tax policy. One of Toomey’s big priorities: “Expensing capital might be the most progrowth element of this exercise,” he told reporters Wednesday. “That’s really really important to me.” Another is to create an incentive to bring home $2.6 trillion in corporate profits sitting overseas.

Orrin Hatch

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

Hatch, who was a second-term senator during the last big tax-code rewrite in 1986, is keenly aware of the issue’s complexity. He declined to say if he believes the Senate will secure a majority to eliminate the state and local tax deduction, a major revenue-raiser targeted in the Trump-GOP framework. “I’m not going to talk about specifics,” he said. “It’s a very complex bill to begin with. And we’re going to have to make some very tough decisions about what we keep and what we don’t.”

Hatch’s panel has a 14-12 split between Republicans and Democrats, which means he can afford to lose no more than one Republican if Democrats decline to play ball.

Susan Collins

Arguably the Senate’s most moderate Republican, the always-meticulous Collins drove a hard bargain on health care that helped torpedo the ACA repeal push. While she has said little about the upcoming 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 a factor: In 2015, she was the only GOP senator of 55 who broke ranks and voted against a budget measure calling for repeal of the estate tax, which the Trump-GOP framework seeks to do. In 2008, she voted for a measure to raise the top tax rate for people earning more than $1 million.

For now, Collins isn’t revealing any of her thoughts on the framework proposed by the Trump administration.

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