No one elected her and you don’t hear much about her, but she’s about to be one of the most important people on Capitol Hill.

Senate Parliamentarian Elizabeth MacDonough can usually be seen perched atop the Senate dais, helping to make sure the Senate floor runs according to the rules. But she’s about to step into the role as arbiter of the Senate’s reconciliation package, where she’ll have the final say in whether a number of key provisions in the House-passed One Big Beautiful Bill Act are in keeping with the Senate’s rules.

Senate Republicans want to make changes to the bill the House passed by a single vote and sent to them. But Senate rules could force a number of changes they find less desirable, too. A veto of any provision by MacDonough could mean major parts of the package are thrown to the wayside, so her rulings will be watched closely by Democrats and Republicans alike in the coming weeks.

MacDonough is responsible for making calls on whether the provisions in the bill are in keeping with the Byrd Rule, named after the late Sen. Robert Byrd, who helped institute the rules governing budget reconciliation packages like President Donald Trump’s “one big, beautiful bill.”

MacDonough has been parliamentarian since 2012 after serving as senior assistant parliamentarian for 10 years. She is the first woman to fill the job since it was created in 1935.

She was called to make several rulings when Democrats used reconciliation to get then-President Joe Biden’s Inflation Reduction Act passed in 2022 as well as the COVID relief package the year before. She also advised Chief Justice John Roberts during Trump’s impeachment trials.

The Byrd Rule

In order for the Senate to use the reconciliation process, which allows it to pass budget packages like this with a simple majority of votes instead of the usual 60 necessary to overcome the Senate’s filibuster, everything in the bill must follow the Byrd Rule.

In the Senate, the process of the Budget Committee reviewing the bill and the parliamentarian to make sure it’s up to snuff is sometimes referred to cheekily as the “Byrd Bath.”

So what are the rules?

The Byrd Rule bars the Senate from including any “extraneous provisions” in budget bills. Anything in the bill, according to the rule, should be necessary to implement the underlying budget resolution that Congress already passed.

Simply stated: If a policy provision doesn’t have an effect on the budget, it can’t be included. Even budget changes that are “merely incidental” to policy provisions are considered out of order.

Now, things are always a bit more complicated in the Senate. The Byrd rule also prohibits Congress from touching Social Security in a reconciliation bill, from increasing the deficit for a fiscal year beyond the period included in the bill, and more. But its basic form is this: everything in the bill must be related to the budget.

It may seem in the weeds, but this review process can have meaningful impacts on reconciliation bills. In the Democrat-backed “Build Back Better” package in 2022, for example, the parliamentarian struck a number of provisions Democrats wanted focused on immigration reform. Democrats ended up having to give up those provisions to pass their package under then-President Joe Biden.

Big policy agenda items that are critical to some Republicans in the One Big Beautiful Bill Act could in theory be slashed out by the parliamentarian, so the process matters.

What does it mean for Trump’s megabill?

There are a number of provisions facing a tenuous path in the Senate because of the Byrd Rule.

Democrats are already vowing to fight policies they say are out of order.

“In the Senate, our committees have been working overtime to prepare for the Byrd Bath, targeting the litany of policies included in the Republican plan that are in clear violation of the reconciliation rules and in some cases, an assault on our very democracy,” Senate Minority Leader Chuck Schumer wrote in a letter to his colleagues Sunday night.

We’ll ultimately have to wait for MacDonough to rule, but if it sounds like policy and not budget, it might be at risk.

Here are a few provisions in the House-passed bill that appear to be potentially at risk of being struck out by the parliamentarian. This is not an exhaustive list and doesn’t account for things that Senate Republicans might want to change or remove from the bill:

AI regulations: The House bill includes language that prohibits state and local governments from enforcing “any law or regulation regulating artificial intelligence models, artificial intelligence systems, or automated decision systems” over a 10-year period.

Federal court provisions: The bill creates a new requirement that could restrict how parties suing the federal government get relief in court. Sen. Joni Ernst, R-Iowa, as recently as Friday suggested at a town hall, when pressed by a constituent about the provision, that it likely wouldn’t pass muster in the Byrd Bath. “I don’t see any argument that could ever be made that this affects mandatory spending or revenues, so I just don’t see that I don’t see that getting into the Senate bills,” Ernst said then.

Planned Parenthood funding ban: The House bill includes a provision that would ban Medicaid funding to Planned Parenthood if it provides abortion services. The parliamentarian stripped a similar provision from a 2017 reconciliation package. It stands to reason she could rule similarly this go-round.