She’s not an elected official and rarely makes headlines, but Senate Parliamentarian Elizabeth MacDonough is about to become one of the most influential figures on Capitol Hill.

As the Senate prepares to take up the House-passed One Big Beautiful Bill Act, all eyes are turning to MacDonough, who will serve as the final authority on whether key provisions in the sweeping reconciliation package comply with Senate rules.

While typically seen quietly presiding over proceedings from atop the Senate dais, MacDonough now faces a high-stakes task: determining which elements of the bill can remain under the complex reconciliation process. Her rulings could dramatically shape—or even dismantle—parts of the legislation, making her decisions critical for both Democrats and Republicans.

Senate Republicans are eager to make changes to the narrowly passed House bill, but the Senate’s strict rules governing reconciliation may force them to accept some provisions—and eliminate others—they might otherwise reject. If MacDonough determines that certain parts of the bill violate Senate procedures, those sections will have to be removed.

At the heart of her role is enforcing the Byrd Rule, named after the late Senator Robert Byrd. The rule limits what can be included in a reconciliation bill, which is designed to fast-track budget-related legislation through the Senate with a simple majority vote—bypassing the usual 60-vote threshold needed to overcome a filibuster.

Under the Byrd Rule, any “extraneous” provisions—those not directly related to budgetary goals or outside the scope of the previously approved budget resolution—must be excluded. The Senate’s review of these provisions is often humorously referred to as the “Byrd Bath.”

MacDonough, who became Senate Parliamentarian in 2012 after a decade as senior assistant, is the first woman to hold the role since it was created in 1935. Her legal expertise and impartial guidance have made her a respected figure in both parties.

She previously made pivotal rulings during Democratic efforts to pass President Joe Biden’s Inflation Reduction Act and COVID relief package through reconciliation. MacDonough also advised Chief Justice John Roberts during both of former President Donald Trump’s impeachment trials.

Now, with another contentious and ambitious bill on the line, MacDonough’s interpretation of Senate rules will be decisive. Whether key priorities survive the “Byrd Bath” will depend on her judgment—making her, once again, a central figure in the legislative process few Americans may know, but all lawmakers must watch.