Swing vote” is a term we often hear when an important case is pending before the Supreme Court. That in itself is a strange modern development. There was a time when we didn’t even think of judges as voting. Electors vote. Legislators vote. Judges rule. Whatever his politics before he came to the bench, a judge was expected to don the black robes and embody justice itself. As Chief Justice John Marshall famously wrote, the Judiciary’s role is “to say what the law is.”

So much for that. The legal realism school of the early 20th century reduced the law from an absolute to just another policy choice, one with which judges were free to disagree. The realists denied that there was any such thing as the neutral application of law, and judges who came after rushed to agree, finding in this relativism the justification for their own desire to rewrite legislation. Instead of pushing back, the legislature shoved more choices their way, resigning themselves to the idea that courts had the final say not just on what the law is, but on what it should be.

For one such swing voter, the media focus was initially not on her jurisprudence but on her status as the first woman on the Supreme Court, as former Newsweek Washington bureau chief Evan Thomas relates in First: Sandra Day O’Connor. Since O’Connor was appointed by Ronald Reagan in 1981, three more women have followed her onto the Court, and it is increasingly unlikely that any Supreme Court will ever again be exclusively male.

O’Connor grew up on a remote Arizona ranch where she learned the self-reliance typical of conservatives in the American West. After graduating from Stanford University’s law school, she famously couldn’t find a job practicing law, as many established firms wouldn’t hire a woman. So she forged her own path, starting a small practice. O’Connor also was a part of the Republican wave that took Arizona out of the Democratic column, getting involved in local politics as an early supporter of Barry Goldwater.

By 1969, she was in elected office, herself, in the Arizona State Senate; three years later, she was the majority leader. Her tenure in the Senate will not seem unfamiliar to scholars of her later jurisprudence. She was conservative, but practical, focused on big ideas but also on tangible results. She naturally pushed for modernization to the state’s laws that would treat women as equal citizens, but also soft-pedalled the proposed Equal Rights Amendment, knowing that there wasn’t adequate support for it in the legislature.

O’Connor was eventually elected a Superior Court judge in Maricopa County. After brushing off entreaties to run for governor, she was elevated to the Arizona Court of Appeals in 1979, where she remained until President Reagan nominated her to the federal bench.

It was a rapid ascent, and all the more remarkable because of the still considerable obstacles in those days to women in the law. (But Reagan had already promised during his campaign that he would appoint a woman to the Court.) As she prepared for the nomination hearings, O’Connor crossed paths with an up-and-coming conservative at the Department of Justice: John G. Roberts. (Roberts would initially be appointed to succeed O’Connor in 2005 before his nomination was shifted to that of Chief Justice following William Rehnquist’s death the same year.) Despite some initial uncertainty about her conservatism, O’Connor was confirmed by a 99-0 vote, with one senator absent.

Abortion, then as now, was a burning issue. Thomas revisits O’Connor’s views on the question throughout the book, and the answer that emerges is confusing. It will likely remain so. O’Connor was never outspoken on social issues, and on this one more than others she kept her true feelings to herself. At her confirmation hearing, she answered questions on Roe v. Wade (1973) in a way that would become increasingly common among judicial nominees: she believed that courts should construe laws, not make them, and that precedents should be upheld except in rare instances.

If that sounds opaque, it was likely meant to be. Her fellow justices did not know what to think of her either, and various factions of the divided court attempted to bring her into their camp. O’Connor, though essentially conservative in philosophy, defied categorization. Her overriding principle was judicial restraint, a welcome reaction to a Court that had expanded its influence for decades by intruding into spheres once reserved for the political branches. But she was also, as one former clerk described it, “always thinking of the real-world consequences.”

On Roe specifically, her theory of incremental change meant that she was never likely to overturn it. In 1989, when Webster v. Reproductive Health Services gave the Court the chance to do so, O’Connor held back from fully joining the four justices who would have been happy to overturn Roe. Instead, she concurred narrowly, voting to uphold the abortion restrictions at issue while affirming the basic holding in Roe. Her opinion was minimalist and did not stray from the specific facts of the case before them, with the result being that a broad ruling on abortion’s constitutionality was impossible.

Three years later, she wrote for a 5-4 majority in Planned Parenthood v. Casey that further entrenched her incrementalist approach, to the frustration of abortion advocates and opponents, who both wished for a final resolution to the vexing debate. For her part, O’Connor believed that any sweeping ruling by the Court was more likely to inflame a fight than to extinguish it. Thomas writes: “O’Connor was very aware of the Court’s obligation to step in and resolve some festering disputes, but she was also sensitive to the need to allow some vexing issues…to evolve over time, albeit with some gentle nudging from the Supreme Court.”

This is the jurisprudence that one of her friends called “consequentialist.” But consequentialist, incrementalist, minimalist, or whatever else you call it, O’Connor’s way of operating is still a conservative variation of the judicial activism of the Warren and Burger Courts. She truly is a political moderate, one who when confronted with a yes-no question is inclined to say “maybe.” This is political restraint, but it is hard to truly call it judicial restraint.

Similarly, John Roberts, a Reagan conservative, has come to find himself in the middle of a divided court, as detailed in The Chief: The Life and Turbulent Times of Chief Justice John Roberts by CNN legal analyst Joan Biskupic.

Roberts was raised in Midwestern suburbia, the son of a steel executive. In a now well-trod career path for Supreme Court justices, he attended an Ivy League school (in his case, Harvard), top-flight law school (Harvard again), and landed an appellate clerkship after graduation. He followed that with a clerkship at the Supreme Court, working for the justice he would one day replace: Rehnquist.

After that, Roberts served as a special assistant for Reagan’s Attorney General, William French Smith, where he worked on a concept that was radical at the time, but is unremarkable today: using the legal resources of the Justice Department to work for court rulings that furthered a conservative legal agenda.

Biskupic focuses much of her narrative on Roberts’s tenure in the Reagan Administration largely out of necessity: it is the only period of his life for which comprehensive records are available. Evan Thomas had access to personal papers of O’Connor and her husband, as well as Court documents and other justices’ memoirs and writings, all of which contributed to a fleshed-out portrayal of his subject’s life and career. Because Roberts is still on the Court, Biskupic lacked these advantages, leaving a skin-and-bones sketch based largely on his activities from ages 26 to 31.

She also, unlike Thomas, shows little effort at objectivity, excoriating the theories advanced in Roberts’s memos from those years. In one bizarre passage, Biskupic bizarrely spends two pages condemning the administration’s position in Bob Jones University v. United States (1983), a case involving whether tax exemptions may be denied to schools that racially discriminate, only to end the section with the enigmatic parenthetical: “Roberts had not participated in the Bob Jones litigation.” The reader is left to wonder why the material was included in his biography in the first place.

After moving from the Justice Department in 1982 to work directly for the president in the White House Counsel’s office, he left government service four years later for Hogan and Hartson, a prominent D.C. law firm. Fitting his expertise and interests, his work was mostly appellate in nature. He appeared frequently before the Supreme Court, including in some pro bono cases, before joining George H.W. Bush’s administration in 1989 as a deputy solicitor general under Ken Starr. Bush nominated Roberts to the D.C. Circuit Court of Appeals in 1992, a high honor for the 37-year-old attorney, but the Democratic Senate declined to act and the nomination expired in January 1993.

After another turn in private practice during the Clinton Administration, the second President Bush was able to make good on his father’s efforts. It took two years, but this time Roberts’s nomination to the D.C. Circuit was confirmed, and in 2003 he took his seat on what is generally regarded as the second-highest court in the land.

The take-no-prisoners seeds that were planted with Robert Bork’s nomination to the high court in 1987 were beginning to find their full flower by 2005 when Roberts was nominated to be the new Chief Justice. In response to Democrats’ fears of a more conservative court, Roberts spoke of a desire for greater judicial consensus. The best way to achieve that, in his view, was through narrow holdings. “I think the narrower the better,” he told law professor and legal affairs writer Jeffrey Rosen—a response in the O’Connor tradition of minimalism.

When Roberts and Samuel Alito took Rehnquist and O’Connor’s places on the Court, it was generally seen as a shift to the right. Biskupic focuses especially on Roberts’s role in affirmative action cases, in which he has departed from O’Connor’s incremental approach. Had Roberts ruled in that vein consistently, it is doubtful that anyone would have considered him a swing vote at all. Obamacare changed all that.

The case of National Federation of Independent Businesses v. Sebelius came to the Court in 2010 and concerned the constitutionality of the Affordable Care Act’s individual mandate and expansion of Medicaid, two parts of that mammoth piece of legislation that its authors deemed crucial for the whole scheme to function. Court-watchers initially eyed Kennedy as the conservative most likely to fall away from the pack, but in their first discussions after the case was argued, the justices lined up 5-4 to strike down the individual mandate, with Kennedy joining Roberts, Antonin Scalia, Clarence Thomas, and Alito. According to Biskupic (none of the justices have ever publicly commented on their deliberations,) Roberts did agree to join the four liberals in upholding the Medicaid expansion.

That might have been a predictable result but, as we know now, it was not to be. In Biskupic’s telling, Roberts wanted a compromise decision that would invalidate the individual mandate without striking down the whole law—a narrow verdict, of the sort he discussed at his confirmation hearings. The way the law was structured made this nearly impossible. In addition to lacking a severability clause (a section of the law that says that if one part is struck down by the courts, the rest remains in force) the massive legislative machinery of Obamacare was all interdependent. Striking down the individual mandate alone would be like removing one gear from an engine: the effects would cause the whole machine to fail.

With a minimalist opinion off the table, Roberts was left with the choice to either strike down the whole law or uphold substantially all of it. The media, Democratic politicians, and even President Obama kept pressure on the Court to align itself with public opinion, preemptively portraying a decision striking down Obamacare as at odds with democracy itself. For Roberts, concerned with the Court’s image and legacy, the pressure was enough to overcome his good sense and constitutional values. He became the swing voter in NFIB v. Sebelius, and swung hard to the left. He claimed that Congress’s interstate commerce rational was unconstitutional, but that the mandate could survive as a tax. Besides doing damage to the already confusing line of cases surrounding direct taxation, Roberts produced a shattered opinion, with the four liberals joining only because it got them the result they sought: they concurred separately to say that the interstate commerce justification was, in fact, correct.

Roberts showed a moderate stripe by also flipping on the part of the law he had been prepared to uphold: Congress coercing the states to expand Medicaid. That part of the ruling may have knock-on effects that benefit conservative federalists in the future. But it shows how invested Roberts had become in the legislative result, rather than in the judicial logic of what he was writing. The Chief Justice had been a stalwart textualist in previous opinions, and mostly has done the same since NFIB v. Sebelius, but his departure from those principles will forever stand out in any assessment of his tenure on the Court.

Biskupic and Thomas both discuss the case of West Coast Hotel Co. v. Parrish, the 1937 decision that gave us the legend of the “switch in time that saved nine.” The story goes that Franklin Roosevelt proposed packing the Court because he was frustrated that they kept striking down his New Deal programs. In response, Justice Owen Roberts (the last name is a nice coincidence) switched his vote in West Coast Hotel, a minimum wage case, to side with the liberals and uphold the law. Once New Deal Democrats saw that the Court had changed, they dropped the court-packing scheme. It’s a fine tale, and both authors cite it uncritically, but legal historians have mostly discredited it.

In NFIB v. Sebelius, on the other hand, the legend has come to life. Roberts had long favored O’Connor’s minimalism; now, he embraced her moderation as well. That he did so only once shows the difference in Roberts and O’Connor as swing voters. O’Connor was happy to legislate from the bench in pursuit of moderate results. There was a minimalist aspect to that moderation—a former office holder herself, she did not want to overturn legislatures with sweeping decisions, believing lawmakers should be the ones to write the law—but in imposing those moderate sentiments through balancing tests and fact-based rulings, she nonetheless put her stamp on the law.

Roberts, on the other hand, doesn’t go out of his way to strike down laws, and he looks for narrow grounds on which to decide cases, but failing that, he is as conservative as Scalia was, and will rule accordingly. The Obamacare case was a rare exception to his typical practice of conservative minimalism and ruling in accordance with the original public understanding of the Constitution.

What does that mean for the Court today? Swing justices are not going away, but the title will change hands from time to time. The results from the 2017-18 term show that Roberts was not the most frequent swing vote in those cases. Neither, surprisingly, was Kennedy, who did not join the Court’s four liberals in even one 5-4 decision the entire term. The justice most likely to swing was Gorsuch.

In part, this shows the futility of trying to portray the Justices as points on a spectrum. Law does not work that way, and views on different constitutional and legal issues may correlate to some extent, but each Justice’s idiosyncratic views will often defy characterization. Yet some, like Roberts and O’Connor, have a measured approach to jurisprudence that makes them more likely to play the role.

Will that change now that Brett Kavanaugh has taken Kennedy’s place on the Court? This term and the next will test the notion of Roberts as a conservative and determine whether he falls into the O’Connor role of moderate and minimalist on a permanent basis.