Senate Republicans are hoping to bring their tax bill to a vote the week after Thanksgiving. But with less than two weeks to go, they can’t seem to agree on what tax reform should look like.

Getting 50 Republican senators to vote for the final bill will be difficult, especially because there are dramatically divergent views on the bill’s goals. This is not just about senators squabbling over small details; there are serious ideological differences that could derail the entire bill.

The first official defection came from Sen. Ron Johnson (R–WI), who released a statement last week saying he could not support the bill as written because it favors large corporations and doesn’t give enough help to small “pass-through” businesses, where business owners pay individual tax on their business’s profits.

“I just have in my heart a real affinity for these owner-operated pass-throughs,” Johnson recently told the New York Times’s Jim Tankersley. “We need to make American businesses competitive — they’re not right now. But in making businesses competitive, we can’t leave behind the pass-throughs.”

Johnson’s defection is noteworthy, but it’s not even the biggest challenge the Senate bill is facing right now.

There are currently two huge obstacles to the Senate bill passing. First, there’s the fact that the massive cuts in both the House and Senate tax bills will increase the national debt by about $1.5 trillion over the next decade. Some conservative senators have made it clear that’s a nonstarter for them.

Then there’s the inclusion of a provision that would repeal Obamacare’s individual mandate tax penalty, which would give Republicans more than $330 billion extra to work with over the next decade, but would also result in about 13 million people losing their health insurance. That could anger moderate Republicans who voted against Obamacare repeal earlier this summer.

In fact, the policy fight over tax reform currently shaping up in Congress is similar to the health care debate in many ways. Lawmakers have an extremely tight deadline and a lot of competing demands. And just like with health care, Senate Republicans have a slim margin of error — they can only afford to lose two votes.

Here are the competing Republican factions on taxes, so far:

The deficit hawks: Bob Corker, Jeff Flake, John McCain

Sens. Bob Corker (R–TN) and Jeff Flake (R–AZ) have made it very clear they don’t think a Republican tax bill should increase the national deficit in any way.

“There are several of us that are trying to figure out a way to make sure this doesn’t hurt us relative to deficits,” Corker told Politico last week. “We’re looking globally at the whole thing and trying to do what we can to make it more fiscally palatable.”

A month ago, Corker was even more blunt, flatly stating he wouldn’t support a bill that would add to the national deficit.

“With realistic growth projections, it cannot produce a deficit,” Corker told Bloomberg last month. “There is no way in hell I’m voting for it.”

Make no mistake: Corker wants to pass a tax bill. But he isn’t seeking reelection next year, so he’s unencumbered by political pressure to hold his nose and vote for something he doesn’t like. Flake is also not seeking reelection, and he’s been clear he doesn’t think tax reform should just be tax cuts. When the House bill was released, Flake took to the Senate floor criticizing steep tax cuts without real tax reform.

“We cannot simply rely on rosy economic assumptions, rosy growth rates to fill in the gap; we’ve got to make tough decisions,” he said. “We’ve been hearing a lot about cuts, cuts, cuts. If we are going to do cuts, cuts, cuts, we have got to do wholesale reform. With the national debt exceeding $20 trillion, we have got to do this seriously.”

Another person to watch is Sen. John McCain, who voted against the Bush tax cuts in 2001 and 2003. McCain, who was reelected in 2016 and likely won’t run again due to his recent brain cancer diagnosis, is also still a stickler for regular order and wants a tax bill passed in a bipartisan way. If it’s not, he may pull the same tactic he did on Obamacare repeal: vote it down.

The moderates: Susan Collins, Lisa Murkowski

Susan Collins (R–ME) and Lisa Murkowski (R-AK) are the two moderate senators whose votes helped doom Obamacare repeal. They’re expected to be friendlier to a tax bill, although both have said they want to see a final version before making up their minds. If history is any indication, Collins voted for the Bush tax cuts in the early 2000s.

But if there’s one thing that will keep her from voting for the current bill, it’s Republicans’ attempt to repeal Obamacare’s individual mandate. Collins said as much Sunday morning on ABC’s This Week.

“But the biggest mistake was putting in a provision from the Affordable Care Act into the Senate bill that’s not in the House bill,” she told host George Stephanopoulos. “I hope that will be dropped.”

But that’s not all; Collins outlined many other things in the Senate bill she’s not happy with, including the fact that tax cuts for the individual are temporary while the ones for corporations are permanent. She also raised concerns about the potential that tax cuts could trigger a $25 billion cut to Medicare.

“That is obviously something that I cannot support,” Collins said. “I do believe that’s going to be dealt with as part of the budget negotiations that are ongoing right now. There are ways to reform our entitlement programs, but that is not one of them.”

In the past, Murkowski has agreed she doesn’t think the tax bill should be tied into health care issues, saying it “may be unnecessarily complicating things.”

But passage of the bipartisan Alexander-Murray bill to stabilize the Obamacare exchanges isn’t a deal breaker for Murkowski. While she supports the bill, she recently released a statement saying that “one should not assume this is a precondition for my support for the tax bill.”

Republican leadership is trying to provide incentives for Murkowski and Collins to get on board with a tax bill. Not repealing the estate tax is seen as an incentive for Collins, while Murkowski is trying to work out a deal that could open up the Arctic National Wildlife Refuge in Alaska to oil and natural gas drilling as a way to add revenue to the tax bill.

Senators pushing for Obamacare’s individual mandate repeal: Ted Cruz, Tom Cotton, and Rand Paul

On the other side of the individual mandate question, we have Sens. Ted Cruz (R–TX) and Tom Cotton (R-AR). The two conservative senators really want to use the tax bill as a chance to repeal the individual mandate tax penalty after the Senate killed its own attempt to repeal and replace Obamacare earlier this year.

Getting rid of the tax penalty could save more than $338 billion over the next 10 years, but it would also lead to 4 million more uninsured Americans by 2019 and 13 million by 2027, according to analysis by the Congressional Budget Office. As Vox’s Dylan Scott explains, getting rid of the mandate would save money because it would mean fewer people would likely enrolled in Medicaid and Obamacare’s insurance marketplaces. And it would satisfy the demands of President Trump, who wants the Affordable Care Act dismantled one way or another.

In addition to wanting action on health care, Cruz has another demand for a tax bill: He’s not a fan of the House plan to repeal the state and local tax deduction, which would effectively raise taxes on people in high-tax states like New York, New Jersey, and California. On Tuesday, Cruz called the provision unfair.

“There are some taxpayers who are losing exemptions, particularly in some high-tax states like New York or California that could conceivably be paying higher taxes,” he said at a recent press conference. “I think that is a mistake. I think tax reform needs to cut taxes for everybody.”

Even so, both Cruz and Cotton probably won’t buck party leadership on taxes, and will likely support the Senate plan.

Child tax credit: Marco Rubio, Mike Lee

Sens. Marco Rubio (R-FL) and Mike Lee (R-UT) have long been pushing for an increase in the child tax credit as a boost to the middle-class and working families. Right now, parents can get a tax credit of up to $1,000 per child under the age of 17, according to the Urban-Brookings Tax Policy Center. But advocates were disappointed by the House plan which would only increase the child tax credit to $1,600. Under Rubio and Lee’s urging, the Senate plan raises it to $2,000.

“When you compare that to some of the tax cuts you’re going to see in other parts of this tax reform, I would say that’s not nearly enough,” Rubio recently said of the House tax credit. “It’s certainly not enough to make a difference.”

Especially given the fact that the House tax plan offers the biggest benefits to big businesses and corporations, and could ultimately raise taxes on middle-class earners, Rubio hasn’t been shy about saying the bill needs to offer something to the middle class too. Increasing the child tax credit is something conservative Republicans like Lee and Rubio and Democrats can agree on. Even though this provision may seem small, it could have huge implications for the tax bill’s chances.

Rand Paul: just being Rand Paul

If there’s a persistent thorn in the side of Senate leadership, it’s libertarian Rand Paul (R–KY), who is known for his penchant of opposing leadership proposals because he believes they don’t go far enough in cutting taxes or federal spending. Paul has been clear he wants a tax cut for businesses and working families but doesn’t think the math is there to do both.

“The danger for this bill right now is the pay-for may be a middle-class tax hike. And if that’s that, it’s going to be a real problem,” he told Politico in a recent interview. “If you lower the taxes on the rich and lower the taxes on the poor and then say, ‘Oh, it’s going to be revenue neutral,’ we’ve got to raise somebody’s taxes to pay for it.”

The Kentucky senator just came back to the Senate and is still healing from six broken ribs, after he was attacked by a neighbor at his Bowling Green home a few weeks ago. The bizarre incident was attributed to a lawn dispute.

Paul has never been afraid of bucking party leadership or the will of President Trump, so he typically tightens Republicans’ margins of error on voting, making it more difficult for them to get to 51 votes.