The Senate parliamentarian just made it more difficult for Republicans to weaken the Affordable Care Act’s protections for pre-existing conditions ― although exactly how difficult remains to be seen.

Parliamentarian Elizabeth MacDonough on Thursday advised senators that a key GOP proposal, allowing insurers leeway to offer less comprehensive health plans, would require 60 votes to pass. The proposal has been part of previous health care bills and was, at least as of Thursday morning, under consideration for inclusion in the legislation GOP leaders hope to pass by Friday. It was the latest in a string of rulings over the last few days that, together, could significantly limit what Republicans can achieve with their narrow, 52-seat majority.

Thursday’s decision from the parliamentarian, as summarized by the Senate Budget Committee’s Democratic staff affiliated with ranking member Bernie Sanders (I-Vt.), focuses on a proposal that would allow states to ignore existing requirements that all insurance policies pay for an “essential” set of benefits. Those benefits include not just hospitalization and prescription drugs, but also mental health and maternity care.

Because that provision is not primarily an effort to change fiscal policy, the parliamentarian determined, it does not conform to the rules of the budget reconciliation process ― a special, expedited method for passing legislation exempt from Senate filibusters, which require 60 votes to overcome.

That provision, known as “1332 waivers,” is one of several that the parliamentarian has ruled out of bounds for reconciliation. Among the others are a proposal that would allow insurers more leeway to vary premiums by age, by as much as a factor of five.

Without those proposals or variants on them, it’s not clear that Republicans can use reconciliation to weaken or eliminate the existing protections for people with pre-existing conditions ― protections that are wildly popular but have also forced insurers to raise underlying premiums, since now the carriers are paying the weighty medical costs that people with significant medical problems generate.

“The parliamentarian’s reported rulings have made clear that the budget reconciliation process is only available for modifying the revenues and expenses of the United States, not for weakening the protections the law provides American health insurance consumers,” Timothy Jost, a law professor at Washington and Lee University and expert on health care legislation, told HuffPost.

To be clear, Republicans don’t have to drop these proposals altogether. But to pass them with less than 60 votes, they would have to find some way of rewriting them to satisfy the rules of reconciliation. Republican policy aides are already working on that, Bloomberg’s Steven Dennis reports, and they say they are optimistic they can succeed.

Their other alternative, one that conservatives like Sen. Ted Cruz (R-Texas) have already raised, would be to overrule the parliamentarian, by having the Senate’s presiding officer disregard the advice.

Other Republicans have said they don’t want to do that, because it would change precedent and render the filibuster irrelevant. But Republicans have already shown that past vows are not always binding, particularly when it comes to their crusade to repeal Obamacare.