Justice David H. Souter's retirement from the Supreme Court will open a window into whether President Obama is truly a coolheaded, moderate technocrat, as claimed by Democrats, or a standard-bearer for some of the left's most extreme ideas, as claimed by Republicans.

The stakes are high - the nine justices set national law on divisive issues such as affirmative action, abortion, and religion. But Obama's choice will not shift the court's balance of power, which is divided among four liberals (John Paul Stevens, Souter, Ruth Bader Ginsburg, and Stephen G. Breyer), four conservatives (John G. Roberts Jr., Antonin Scalia, Clarence Thomas, and Samuel A. Alito Jr.), and one man in the middle (Anthony M. Kennedy). Obama's nominee will keep the liberal pack intact.

But liberals want more. They want a progressive thinker who can challenge Scalia and Thomas and their revolutionary ideas for modern constitutional law. While a witty man and elegant writer, Souter did not pen any landmark opinions that will be remembered in the decades to come. The liberal wing of the court has yet to produce a worthy heir to the mantle of Chief Justice Earl Warren or Justice William Brennan.

Obama promises something different. In 2007, candidate Obama declared that his judges would "recognize what it's like to be a young teenage mom, the empathy to understand what it's like to be poor or African American or gay or disabled or old." When he announced Souter's retirement, the president stated he would nominate "someone who understands that justice isn't about some abstract legal theory or footnote in a case book; it is also about how our laws affect the daily realities of people's lives." Empathy is "an essential ingredient for arriving at just decisions and outcomes."

In his 2005 confirmation hearings, Roberts compared judges to neutral umpires in a baseball game. Sen. Obama did not vote to confirm Roberts or Alito, but now proposes to appoint a Great Empathizer who will call balls and strikes with a strike zone that depends on the sex, race, and social and economic background of the players. Nothing could be more damaging to the fairness of the game, or to the idea of a rule of law that is blind to the identity of the parties before it.

Empathy has a proper place in other areas of life, such as medicine or charitable work. And the law does take account of a party's identity when necessary - in deciding whether someone has suffered racial or gender discrimination, for example. But judges should not apply these rules differently in individual cases because of the skin color, or sex, or religion of the plaintiff or defendant.

Obama's call for emotive judges contradicts his moderate campaign positions. As a candidate, Obama declared that marriage should occur only between a man and a woman, and he refused to endorse the California Supreme Court's blessing of gay marriage. He agreed that the Second Amendment protected an individual's right to gun ownership, a stance rejected by the four liberal justices on the Supreme Court last year. Candidate Obama attacked the view of the four liberals and Kennedy in last year's decision prohibiting the death penalty for child rapists.

During the campaign, Obama spoke movingly on the role of religion in public life, putting him at odds with liberal justices who have sought to drive even symbolic invocations of religion out of the public sphere. Obama's very election signals a new and welcome American attitude on race, but it runs counter to the liberals' uncompromising defense of racial preferences in school admissions, government contracting, and voting rights.

A justice who followed candidate Obama's views would come from a very different list from one who went beyond cases and footnotes to reach for a higher state of empathy. Yet the judicial mold created by the candidate would be more beneficial to the president, whose ambitious economic and political program argues for a justice with a more modest, neutral vision of his role.

With solid majorities in Congress, Obama's policies face little meaningful Republican opposition. Therefore, he should seek judges who will give the bills of his Congress as little trouble as possible, which, ironically, counsels in favor of justices who don't believe in judicial activism.

Franklin Roosevelt faced exactly this dilemma. With large majorities at his back, FDR pushed through sweeping legislative efforts to end the Great Depression (which never really worked). His only obstacle became the Supreme Court, which held several basic New Deal laws to violate the Constitution's limits on federal powers and the economic rights of the individual. Only after FDR waged a campaign to increase the size of the court and give himself more appointments did the justices surrender. The New Deal could not have survived without judges that deferred to the legislature on economic regulation.

Obama could make a pick based solely on race or sex - though it's not clear why the most empathetic judges are minorities or women - to please parts of his coalition. But if the president wants to secure the success of his economic, political, and national-security objectives, he should remember FDR's example and choose a judge who believes in the right of the president and Congress, not the courts, to make the nation's policies. If Obama shoots for empathy instead, he will give Senate Republicans yet another opportunity to rally around a unifying issue where they better represent the majority of Americans.