Arcane Senate rules could prove a major obstacle to congressional Republicans’ plans to repeal and replace the Affordable Care Act.

Republicans plan to use a process called budget reconciliation to pass their new health bill. The process requires all changes to have a direct impact on the federal budget, significantly constraining what can be in the replacement plan.

There are multiple provisions in the replacement plan that are unlikely to meet that requirement, according to two congressional procedure experts interviewed by Vox. This would leave Republicans in a bind: either break with decades of Senate procedure or let their health bill die.

The provisions that might not qualify include Republicans’ plan to encourage healthy people to buy insurance — crucial to spreading the risk around and making insurance markets work — and perhaps even the provision to defund Planned Parenthood.

The reconciliation process is messy. But it has one big advantage: Legislation passed this way cannot be filibustered. So instead of coming up with a 60-vote majority, Republicans only need 51 votes to pass a reconciliation bill for repeal.

With 52 seats in the Senate, Republicans could plausibly pass their bill without any support from Democrats — making reconciliation an alluring if frustrating approach.

Republicans will need to prove that all parts of the law “directly” affect the federal budget — and that will be tough

When the health care overhaul reaches the Senate floor, Democratic senators can challenge provisions they think do not directly relate to the federal budget. The Senate parliamentarian — who oversees Senate procedure — will issue guidance on each policy, and her rulings can be difficult to predict. This is known as a “Byrd bath,” named in honor of former Sen. Robert Byrd, who pushed for the provision in the 1980s.

“There’s no black-and-white rule. It’s really subject to the parliamentarian’s interpretation,” says Sarah Binder, a congressional procedure expert at George Washington University.

But Binder and one other congressional procedure expert, Greg Koger at the University of Miami, do see certain policies that would be really hard to defend as directly affecting the federal budget.

A surcharge for people who don’t maintain continuous coverage. AHCA includes a 30 percent premium hike for those who buy individual coverage after going without health insurance for at least two months. The surcharge is paid directly to the insurer, making it tough to argue that this provision is about federal spending. Republicans really need this part of the AHCA, too, to encourage healthy Americans to buy plans.

Allowing insurers to charge the oldest enrollees five times as much as the youngest. Republicans want to allow insurers to charge older Americans higher premiums and younger ones lower premiums. This runs into the same issue as the continuous coverage requirement: It appears to primarily be about insurance regulation rather than budgetary issues.

“This falls into an area where it’s not clear to me how it gets through, if it’s telling insurance companies how to set their pricing on insurance,” Binder says.

Defunding Planned Parenthood? The AHCA would, according to my colleague Emily Crockett, “bar Planned Parenthood from receiving hundreds of millions of dollars in federal Medicaid reimbursements — the majority of the organization’s federal funding.” This one is a bit murkier, as it does involve federal spending. But Koger thinks Democrats could easily launch an argument for stripping it out of the bill.

“If I were on the GOP side, I would argue that defunding Planned Parenthood would change Medicaid spending,” says Koger. “But if I were in on the Democratic side, I would say that the change in spending is just incidental, that the real point is to cut out one specific Medicaid provider.”

Republicans do have a nuclear option: not listening to the Senate parliamentarian

The budget reconciliation process is a three-step process that starts with a budget resolution — essentially a list of spending targets for the coming years — and a set of instructions for how committees can hit those targets. In this case, they’d hit them by repealing Obamacare.

The reconciliation process started in January when the Senate passed the budget resolution. Senate committees are now in the midst of the second step: working on legislation that could hit the targets. The process ends on the Senate floor, where Democrats can challenge provisions and get a ruling from the parliamentarian.

One wrinkle in the whole process is that while the Senate parliamentarian does give guidance on what provisions violate reconciliation rules, the final decision rests with the majority party. Republicans could decide they don’t like her advice, and keep their bill intact.

Sen. Ted Cruz (R-TX) has advocated for this approach. “If the parliamentarian disagrees, the vice president has the statutory and constitutional authority (as does the Senate majority) to rule to the contrary,” he wrote last week at Politico. “And that’s exactly what should happen, if necessary.”

But this would be a major break with precedent. Both Binder and Koger said it was incredibly rare for the parliamentarian to be overruled. Binder couldn’t think of a recent example. If Republicans did take Cruz’s approach, it would be risky. Democrats would presumably do the same thing when they eventually come into the majority.

“It is a norm,” says Koger. “And everybody understands there are costs to violating the norms.”