What is the Senate parliamentarian? Lately, she’s been Mitch McConnell’s nemesis.

In recent weeks, Republicans in the Senate have struggled to put together a piece of legislation repealing the Affordable Care Act they can all agree on.

One reason Republicans can’t move forward is that conservatives and more moderate members can’t agree on how much of the Affordable Care Act to scrap.

Another reason is Elizabeth MacDonough.

MacDonough is the Senate Parliamentarian, essentially the body’s referee who usually tends to fly under the radar. This role ensures that the Senate’s expansive, arcane set of rules are followed to the letter.

The position has existed officially since 1935, according to the Senate Historical Office. The first parliamentarian was Charles Watkins, who began working in the Senate as a stenographer in 1904. In 1933, Watkins, who had a photographic memory and was deeply interested with the body’s rules and procedures, became assistant secretary of the Senate. In 1935, at the height of the New Deal, the Senate decided it needed someone to keep track of its procedures and chose Watkins for the job.

More recently, MacDonough has stymied GOP efforts to repeal the Affordable Care Act by reminding them that the Senate only allows certain changes to be passed through “budget reconciliation” – the process Senate Majority Leader Mitch McConnell is trying to use instead of passing the bill normally, which would require 60 votes.

Budget reconciliation requires only 50 votes, which Republicans have. But it’s not designed to be used the way Republicans want to use it, according to former parliamentarian Alan Frumin.

“Reconciliation was designed to bring about discrete budgetary changes at the margins to create changes in revenues and changes in outlays,” Frumin, who served as parliamentarian for each presidential administration from Ronald Regan to Barack Obama, told NPR. “It was not designed for major policy changes,” he said.

The Byrd Rule, named for former senator Robert Byrd, prevents changes that aren’t budgetary in nature from being passed through budget reconciliation.

MacDonough said that several planks of the Senate bill, including the part defunding Planned Parenthood, don’t qualify.

But does the Senate have to abide by the parliamentarian’s ruling? There’s precedent for the parliamentarian to be ignored. In 1975, Vice President Nelson Rockefeller ignored the parliamentarian’s advice over a point of order, allowing a simple majority vote over whether to end a debate. The vice president also serves as president of the Senate.

“In making his controversial ruling, Rockefeller had notified the Senate parliamentarian that he was making the decision on his own, contrary to the parliamentarian’s advice,” the Senate Historical Office’s profile of Rockefeller says.