As I read that letter, I thought about what would have happened if the current Supreme Court were transported back to decide Brown. Two years of deliberation? No way. Unanimous or even near-unanimous decision? Forget it. The decision would have been 5-4 the other way, with Chief Justice John Roberts writing for the majority, "The way to stop discrimination on the basis of race is to stop discriminating on the basis of race"—leaving separate but equal as the standard. The idea that finding unanimity or near-unanimity was important for the fabric of the society would never have come up.

Recent analyses have underscored the new reality of today's Supreme Court: It is polarized along partisan lines in a way that parallels other political institutions and the rest of society, in a fashion we have never seen. A couple of years ago, David Paul Kuhn, writing here, noted that the percentage of rulings by one-vote margins is higher under Roberts than any previous chief justice in American history. Of course, many decisions are unanimous—but it is the tough, divisive, and most important ones that end up with the one-vote margins.

The New York Times's Adam Liptak weighed in recently with a piece called "The Polarized Court," in which he said, "For the first time, the Supreme Court is closely divided along party lines." Scott Lemieux, in The Week, noted further that the polarization on the Court, like the polarization in Congress, is asymmetric; conservative justices have moved very sharply to the right, liberals a bit more modestly to the left. Much of the movement did occur before Roberts was elevated to the Supreme Court, but his leadership has sharpened the divisions much more, on issues ranging from race and voting rights to campaign finance and corporate power.

How did we get here? As politics have become polarized and as two-party competition intensified, control of the courts—which are increasingly making major policy decisions—became more important. With lifetime appointments, a party in power for two or four years could have sway over policy for decades after it left power. But to ensure that sway meant picking judges who were virtual locks to rule the way the party in power wanted. That meant track records in judicial opinions, and that in turn meant choosing sitting judges to move up to the Supreme Court. It also meant choosing younger individuals with more ideology and less seasoning; better to have a justice serving for 30 years or more than for 20 or less.

The Warren Court that decided Brown had five members who had been elected to office—three former U.S. senators, one of whom had also been mayor of Cleveland; one state legislator; and one governor. They were mature, they understood the law, but also understood politics and the impact of their decisions on society. As a consequence, they did not always vote in predictable fashion. Only one of the justices, Sherman Minton, had served on a U.S. appellate court—and he had been a senator before that appointment.