The inner workings of the Supreme Court aren’t usually gripping prime-time viewing. But on the evening of December 12, 2000, the Court dominated headlines as it ended one of the United States’ most contentious elections. And all eyes were on one justice: Sandra Day O’Connor.

This time it wasn’t because she was breaking boundaries—as the first woman to serve on the Supreme Court, she had smashed the glass ceiling almost 20 years before. In this case, the focus was on O’Connor’s coveted “swing vote,” a moderate political stance she used to break ties and, in the case of the contentious Bush v. Gore race, help decide elections. O’Connor’s use of her swing vote earned her a reputation as one of the Court’s most powerful justices, and on that day she was facing one of the highest stakes decisions of her career.

The term “swing vote” refers to a justice whose political views fall on the middle of the spectrum of the rest of the Supreme Court. Because such a justice could swing left or right, their vote is considered a critical tiebreaker.

Over the course of her 24-year-long career, O’Connor’s swing vote was an important one. She broke ties in a number of high-profile cases like Grutter v. Bollinger, which upheld affirmative action in higher education as constitutional, and Planned Parenthood of Southern Pennsylvania v. Casey, which upheld Roe v. Wade and a woman’s right to choose to have an abortion. And in 2000, O’Connor cast the decisive vote in what was arguably the most important case of her tenure—Bush v. Gore.

It was no ordinary lawsuit. The case tested the very stability of the American electoral system, and its winner would go on to become president of the United States. Election night 2000 was chaotic and confusing. News channels swung between declaring Democrat Al Gore and Republican George W. Bush the winners of the election.

When Bush won Florida by just 1,784 votes, the razor-thin margin prompted an automatic recount per state law. That recount shrank the margin even more, reducing Bush’s lead to a mere 327 votes. After a flurry of lawsuits, the Florida Supreme Court weighed in, ruling for manual recounts throughout the state. Bush’s legal team appealed to the Supreme Court, asking for the recounts to stop and arguing that Florida was reaching beyond its election authority.

The stakes were high—nothing less than the legitimacy of the electoral system hung in the balance. And for O’Connor, the personal stakes were high, too. A Republican, she had repeatedly said that she’d only retire if a Republican came into office during the 2000 election so that her replacement would also be a Republican. When O’Connor heard a proclamation that Gore had won the election during an election-night party, she turned away from the television in distress.

“She’s very disappointed because she was hoping to retire,” her husband reportedly told other partygoers. However, notes legal scholar Richard K. Neumann, Jr., this version of the party story is contested, with others suggesting that O’Connor was upset because the election was already being called while polls in other time zones were still open.

Regardless of this potential bias, O’Connor still had to weigh in on Florida’s decision to order manual recounts throughout the state. She took a strong anti-recount stance and joined the majority for a 7-2 ruling that the recounts violated the 14th Amendment’s Equal Protection Clause. She wrote a brief along with Justice Anthony Kennedy, objecting to what both saw as the state’s unequal treatment of the recount in different counties. O’Connor also voted with the 5-4 majority to stop the recounts, breaking a tie in an apparently contentious court. The decision effectively declared Bush president and ended a nearly two-month-long standoff over the election results.

2000 Presidential Election
Chris Hondros/Getty Images
Newspapers announcing the results of the U.S. Supreme Court's decision to halt the Florida ballot recount, claiming George W. Bush as the victor of the 2000 presidential election.

The specifics of the Supreme Court’s inner workings will always be murky to the general public, and there’s no way to know what really motivated O’Connor to cast the deciding vote against Gore. But the Bush v. Gore decision was even more mysterious. “Nothing about this case, Bush v. Gore, No. 00-949, was ordinary,” wrote Linda Greenhouse for the New York Times. “Not its context, not its acceptance over the weekend, not the enormously accelerated schedule with argument on Monday, and not the way the decision was released to the public tonight.”

The decision sealed O’Connor’s legacy as a swing vote. (She retired in 2006.) But despite the influence she wielded in the court, O’Connor never liked the concept. “I don't like that term,” she told Fresh Air’s Terry Gross in 2013. “I never did, and it's not one that I like any better today. I don't think any justice—and I hope I was not one—would swing back and forth and just try to make decisions not based on legal principles but on where you thought the direction should go, and so I never liked that term.”

Like it or not, though, O’Connor's moderate political positions gave her the power to break ties, transcend disagreements and even decide elections.