Republican leaders have been considering using a budget tool known as reconciliation to strip Planned Parenthood of its federal funding.

Reconciliation is a tactic that allows legislation to pass with just 51 votes.

However, the plan has earned the ire of conservatives, who argue the procedure should be used to roll back Obamacare.

Now, a scholar at the Brookings Institution, a left-of-center think tank, is joining conservatives questioning whether reconciliation can be used to defund Planned Parenthood.

Molly Reynolds, a fellow in governance studies at Brookings, explained in a blog post last week that the Byrd Rule could prevent reconciliation from being used to address the group’s federal funding.

“Putting aside whether conservative Republicans would accept the promise of future action in exchange for cooperation now and looking ahead, the Senate’s Byrd Rule will almost certainly come into play,” Reynolds wrote.

The Byrd Rule, adopted temporarily in 1985 and made permanent in 1990, gives senators the chance to object to legislation during the reconciliation process that doesn’t change spending or revenues. The Byrd Rule was enacted to ensure reconciliation is being used to reduce the deficit, and senators can attempt to block a provision within a reconciliation bill on the grounds that it includes “extraneous matter.”

A provision of a reconciliation bill can be ruled “extraneous” if it falls under one of six different definitions, one of which questions if a provision “produces a change in outlays or revenue which is merely incidental to the non-budgetary components of the provision,” meaning that the budgetary component must outweigh the non-budgetary component.

The “merely incidental” test, Reynolds said, is subjective and up to the discretion of the Senate parliamentarian, currently Elizabeth MacDonough.

>>> Commentary: Why Reconciliation Can’t Be Used to Defund Planned Parenthood

If a senator objects to a provision of the reconciliation bill on the grounds that it violates the Byrd Rule, the offending language can be removed from the bill.

The Senate can waive the Byrd Rule, but doing so requires 60 votes.

“What does all this mean for a hypothetical Planned Parenthood provision?” Reynolds wrote. “Actual objections raised on the floor alleging ‘merely incidental’ budget effects are relatively rare, but that obscures any deterrent effects on the rule; after all, legislators may simply avoid including problematic provisions, knowing the parliamentarian may strike them down.”

In addition to passing the “merely incidental” test, Reynolds also said that using reconciliation to defund Planned Parenthood may be problematic because of the type of money the organization receives from the government.

The funds Planned Parenthood receives, such as Title X, qualify as discretionary spending, meaning they are allocated to specific agencies through appropriations acts. Discretionary funds cannot be changed in a reconciliation bill.

Reynolds noted that this specific provision of the Byrd Rule would limit Congress’ ability to cut Planned Parenthood’s discretionary funds—which includes $60 million the group receives through Title X —through reconciliation.

“Again,” Reynolds said, “it’s difficult to predict exactly where the current parliamentarian might come down on these questions. As congressional Republicans continue to plan their budget strategy, however, they’d be wise to recall the words of parliamentarians past and consider how their motives might be read.”

Republicans in Congress have been calling for Planned Parenthood to be stripped of its federal funding following the release of a series of undercover videos from the Center for Medical Progress. The videos show Planned Parenthood officials allegedly discussing the sale of organs and tissue from aborted babies.

Following the release of the videos, lawmakers have begun to hold hearings to investigate the group. Now, Republicans are looking to the upcoming debate on the budget to defund Planned Parenthood.

>>> Conservatives Face Uphill Battle to Use Government Funding Bill to Punish Planned Parenthood