A few years before he retired last summer, Supreme Court Justice John Paul Stevens remarked that since 1971, every one of the justices appointed to the Court—he included himself—was more conservative than the justice who was replaced. He allowed there was room for debate about Justice Ruth Bader Ginsburg, and Stevens made his comment before President Obama’s recent appointments. But the big picture is unmistakable. In the 40 years between Lyndon Johnson and Barack Obama, the Supreme Court moved uniformly and inexorably to the right.

There is also no mistaking the consequences. The great liberal decisions of the Warren Court seem far away, not just because so many years have passed. According to some liberals, there have been no truly progressive justices on the Court for many years—only moderates and conservatives. Conservatives have won important battles, most recently in striking down gun-control laws and limits on the corporate funding of political campaigns, and the Court may play an even more prominent role in the future. The Voting Rights Act, probably the most important law of the civil rights era, has a bull’s-eye on its back, and there is a concerted attack on health-care reform.

Meanwhile, it is not even clear what a liberal agenda for the Supreme Court would look like. The policy initiatives that are most important to progressives today—reforming health care, protecting the environment, improving education, addressing economic inequality—require legislative, not judicial, action. Some causes that were paramount to the Warren Court, like racial justice and the rights of criminal defendants, either have taken on a new and more complex form or have been left behind by many liberals. Other issues—rationalizing immigration policy, protecting individual rights while dealing with the threat of terrorism—have a court-centered component but are more complex, morally and institutionally, than, say, the attack on racial segregation during the civil rights era.

Justice Brennan—with the simple subtitle “Liberal Champion”—does not tell liberals how to deal with a conservative Court, or what the progressive judicial agenda might be today. But it does give us, with depth and clarity, a picture of an era when progressive ideas were ascendant on the Court—and of the conservative reaction precipitated by that era. Justice Brennan was, as the book makes clear, central to the Warren Court from the time he joined it in 1956 until Chief Justice Earl Warren’s retirement in 1969. During that period, the Warren Court expanded the rights of marginalized members of society: African Americans in the segregated South; political dissidents; criminal defendants; city dwellers who were all but disenfranchised by malapportioned legislatures; even welfare recipients. Brennan then tried to carry forward what he saw as the work of the Warren Court during the increasingly conservative years that followed. He had occasional victories but often faced frustration and defeat, leading the authors to delicately raise the question of whether Brennan and the Warren Court had overreached.

Justice Brennan gave one of the co-authors—Stephen Wermiel, then the Supreme Court reporter for The Wall Street Journal—extraordinary access during his last years on the Court. Brennan sat for extensive and candid interviews, and he allowed Wermiel to examine his papers on the condition that Wermiel not publish until Brennan retired. Brennan left the bench in 1990 and died in 1997, but the book was not completed until after Seth Stern, a lawyer and a reporter for Congressional Quarterly, joined the project in 2006.

The finished product is an engaging book, smart, fair, and well-paced. You don’t have to be a lawyer to read it; the book handles legal intricacies with a light touch (but without oversimplifying, a considerable accomplishment). Justice Brennan is more about the small-group dynamics of the Supreme Court than it is about legal doctrine, because Stern and Wermiel emphasize, appropriately, the relationships among the justices—most of whom come across as seriously engaged and conscientious, although not free of egoism and idiosyncrasy. The authors are also more concerned with telling Brennan’s story than with working out its implications. Was the Warren Court a liberal power grab by judges who were fatally out of touch with popular sentiment? Was it a one-time response to the particular circumstances of the 1950s and ’60s, never to be repeated? To the extent the authors address these questions, they offer something of a tentative, middle-of-the-road view: that Brennan’s brand of liberalism was wholly justified for a time, but in the end helped cause the backlash that led to Richard Nixon and, eventually, Ronald Reagan.

Brennan was one of the most important judges in American history—his ideological opposite, Justice Antonin Scalia, has described him as probably the most influential Supreme Court justice of the twentieth century—but he was not interesting in the ways that ordinarily make for a must-read biography. His personal life was mostly conventional. He seems to have instinctively recoiled from introspection or self-revelation. And he spent his professional life reading briefs, negotiating with other judges, and writing opinions—not the stuff of high drama.

William Brennan was born in Newark, New Jersey to parents who had emigrated from Ireland and met in the United States. His father was a manual laborer with a primary-school education who became a union leader and then a popular local politician. Brennan was raised as a Catholic, but not, it seems, in an especially rigorous way. “While he continued to attend Sunday school,” Stern and Wermiel report, “he had happier memories of playing pool in the church’s recreation room for teenagers.” Brennan’s Catholicism remained a minor theme throughout his public life—when he ruled in school prayer and abortion cases, for example.

Brennan was a successful lawyer in private practice, primarily on the management side in labor disputes—a surprising role for the son of a union leader, but an early indication of Brennan’s ability to compartmentalize his personal and professional lives. That same ability showed up in his support for abortion rights, despite his personal views about abortion, and in his unfortunate reluctance to hire a woman as a law clerk, despite his vigorous advocacy of gender equality. Brennan’s prominence as a lawyer, and his active involvement in a movement to reform the administration of New Jersey’s courts, led first to an appointment to the trial court in New Jersey and then, in 1952, to the New Jersey Supreme Court. To the extent he had identifiable philosophical leanings, they were mildly liberal, but no more than that. He made a notable speech denouncing Senator Joseph McCarthy. But there was nothing in his New Jersey career that made him an obvious candidate for either the U.S. Supreme Court or the title of “Liberal Champion.”

His appointment to the Supreme Court was, in fact, a classic example of being in the right place at the right time. In 1956, when Justice Sherman Minton died, President Dwight Eisenhower was in the midst of what he mistakenly thought was a difficult re-election campaign against Adlai Stevenson. Both the northeastern states and Catholic voters were important swing blocs. Eisenhower told his attorney general, Herbert Brownell, to find, if possible, a Catholic Democrat from a swing northeastern state who was relatively young and had significant experience as a judge. There were not many people besides Brennan, 50 years old at the time, who satisfied those criteria. In the Senate committee hearings, Brennan encountered opposition from only two sources: McCarthy, no doubt because of Brennan’s earlier speech; and a group that professed concern that Brennan would follow Roman Catholic doctrine instead of the law. In the end, only McCarthy voted against confirming him.

Brennan’s career on the Court can be divided into two phases—one of triumph, and one of decline. Warren trusted Brennan from the start, and he used his prerogatives as chief justice to put Brennan in an influential position from the beginning of his tenure. Warren was a gifted politician—he had been elected governor of California three times—and he knew what he was doing when he made Brennan his lieutenant. Brennan became famous for his ability to assemble coalitions to support his positions. Stern and Wermiel recount the often-told story of how Brennan asked each new group of law clerks to identify the most important thing they would learn during their clerkship. The puzzled clerks “would watch quizzically as Brennan held up five fingers.” With five votes, he said, you can do anything on the Supreme Court.

The story sounds cynical, and Brennan resented the image of a back-slapping Irish pol who got his way by charm. He was, in fact, legendary for being gregarious and affable, and he was strikingly free of vanity and pettiness. Those qualities undoubtedly helped him assemble coalitions within the Court. But Stern and Wermiel show that Brennan’s success was attributable to something much more substantial: He took his colleagues’ ideas and concerns seriously. He was not rigid or self-righteous about his own views. The fair criticism of Brennan is that his opinions, written to satisfy others and secure those five votes, sometimes lack clarity or principled consistency (although less often than one might think, given Brennan’s reputation). In any event, the way in which Brennan got to five was much more a matter of high politics, characterized by genuine intellectual commitment and accommodation, than of horse-trading or personal conviviality.

Brennan wrote (or sometimes ghostwrote, under Warren’s name) important opinions on the principal project of the Warren Court: the battle against Jim Crow apartheid. In 1962, he wrote the opinion that paved the way for the principle of “one person, one vote”—the rule, intensely controversial at the time, that state legislative and congressional districts have to be equal in population. That rule ended the gross underrepresentation of urban areas in state legislatures. Two years later, Brennan wrote the Court’s opinion in New York Times v. Sullivan, arguably the single most important First Amendment decision in American history. The case limited the circumstances in which public officials can sue for defamation and, more importantly, established more clearly than any other case had the principle that the central purpose of the First Amendment is to protect political dissent. When the Warren Court ordered an end to state-sponsored prayer in public schools, Brennan wrote a nuanced concurring opinion to try to define the proper role of religion in public life—an opinion, Stern and Wermiel suggest, that was intended as much as anything to explain his position to the Catholic Church.

Probably the Warren Court’s most widely reviled decisions were those that redefined the rights of criminal defendants; Brennan wrote key opinions that increased the access of state criminal defendants to federal court, an important step in making those rights enforceable. In 1969, at the very end of Warren’s tenure as chief justice, Brennan wrote an opinion holding that a state violated the Constitution when it denied welfare benefits to people who had not lived in the state for a year. That decision in some ways was the high-water mark of the Warren Court (too high for the chief justice himself, who dissented), and the opinion was characteristic of Brennan. It emphasized the “right to travel” from state to state; not coincidentally, some of the key swing justices were committed to a right to travel. But at the same time, the opinion planted the seed of a right to a minimum level of economic support from the state. The seed did not flourish, though.

The Warren Court ended abruptly. In the 1968 presidential campaign, Richard Nixon attacked the Court, particularly for its rulings on criminal defendants’ rights. Nixon then made four appointments—including Warren’s replacement—in his first three years in office. That was the beginning of the conservative avalanche that Stevens described. In the next 20 years of his career, Brennan often found himself playing defense, trying to preserve the Warren legacy against an increasingly conservative Court.

Still, Brennan had his share of success. He wrote an important opinion ruling that welfare recipients have the right to a hearing before their benefits are terminated. He united a majority of the Court behind a constitutional rule forbidding sex discrimination. He established the principle that citizens can sue for damages if federal officials violate their constitutional rights. He was a pivotal figure in protecting the ability of both governments and private firms to engage in affirmative action to benefit racial minorities. He played a role—relatively minor, but not insignificant—in establishing what turned out to be the most controversial right of all, the right to an abortion announced by Roe v. Wade. In a flashback to his Warren Court days, he cobbled together five votes to strike down a Texas law that denied free public education to children who were undocumented aliens. And he wrote the opinions that declared unconstitutional, on First Amendment grounds, laws forbidding the burning of the American flag.

But in this second phase of his career, Brennan was very often in dissent. He grew increasingly frustrated at the Court’s conservatism, and it showed in his opinions. He issued sharply written dissents that risked alienating his new colleagues, something the Brennan of the Warren Court years would not have done. His liberalism became more reflexive: He reserved some of his harshest rhetoric to defend Warren Court rulings, particularly about criminal defendants’ rights, that were crucial in the 1960s, when criminal justice in some states was primitive at best and racist at worst, but which had become more reasonably disputable in the 1980s, when most states had cleaned up their act.

Brennan’s handling of capital punishment cases became a signature of his last decades on the Court. In 1972, the Court, in a splintered series of opinions, effectively held that the death penalty as then administered in the United States was so unevenly applied that it violated the Eighth Amendment’s ban on “cruel and unusual punishments.” But in 1976, after 37 states re-enacted death penalty laws with provisions that addressed the Court’s objections, a majority of the Court backed down and effectively reinstated the death penalty. Brennan had come to the conclusion that the death penalty was unconstitutional in all circumstances, and he vowed never to uphold a death sentence again. He was true to his word—in every death penalty case that reached the Court, Brennan filed an opinion reasserting his view. It was an act of conscience, but it raised serious questions about a justice’s obligation to follow precedent, and, perhaps more than anything else, it branded Brennan as a champion of lost liberal causes, out of touch with his Court and his society.

The latter part of Justice Brennan’s career raises a recurring question for progressives. Is it better to continue to assert principles traditionally associated with liberalism, even when they have become unpopular and have little chance of prevailing? Or should progressives instead try to find common ground with ideological opponents, and entertain the possibility that times have changed in a way that makes the old liberal commitments less imperative or even obsolete? By the end of his career, Brennan was far from the center of the action, certainly further than he would have been if he had continued to show the kind of flexibility and openness that characterized his work on the Warren Court.

The first part of Brennan’s career teaches quite a different lesson. A common trope is to condemn both the “liberal activism” of the Warren Court and the “conservative activism” of later Supreme Courts, as if the two forms of “activism” were on par. But the striking thing about Brennan’s work on the Warren Court is how the decisions of that era—which provoked red-hot controversy at the time—have become universally accepted. No one today will argue that Jim Crow-style school segregation is constitutional. “One person, one vote” has become so thoroughly settled that, for most people, it is hard to see how things could be otherwise. The specifics of New York Times v. Sullivan can be debated, but its central thrust—protecting political dissent is the heart of the First Amendment—is beyond question. An increasingly conservative Court has, in the most important instances, preserved Warren Court decisions on school prayer and criminal defendants’ rights, even when there were opportunities to cut them back. If you ask critics of the Warren Court to name a decision that they’d like to overturn, the usual answer is Roe v. Wade. But whatever its merits, Roe was decided three years after Earl Warren retired, by a Court that included four appointees of Richard Nixon, who ran against the Warren Court in 1968. (Three of the Nixon four were in the seven-justice majority.) The reputation of the Warren Court as “activist” rests on decisions that have become not just accepted but celebrated.

There is a reason that those decisions are celebrated. They reflect a constitutional vision that, however imperfectly articulated, resonates with the deepest commitments of the American system of government. The Warren Court’s project was to protect the people who do not get a fair shake in the democratic process. People and groups who can protect their own interests by voting do not need—and should not get—extra protection from the life-tenured justices of the Supreme Court. But African Americans in the Jim Crow South, political dissidents, criminal defendants in unfairly rigged systems of justice, city dwellers in a state where a minority of voters has locked up control of the legislature—those are the people whom the courts should protect. William Brennan came to understand this vision of the Constitution instinctively, and he, probably more than anyone, was able to make it the animating spirit of a remarkable era in the history of the Supreme Court. If that is what it means to be a Liberal Champion, then liberals should be proud.