Many American presidents have been lawyers, but almost none have come to office with Barack Obama’s knowledge of the Supreme Court. Before he was 30, he was editing articles by eminent legal scholars on the court’s decisions. Later, as a law professor, he led students through landmark cases from Plessy v. Ferguson to Bush v. Gore. (He sometimes shared his own copies, marked with emphatic underlines and notes in bold, all-caps script.)

Now Mr. Obama is preparing to select his first Supreme Court nominee to replace retiring Justice David H. Souter. In interviews, former colleagues and students say they have a fairly strong sense of the kind of justice he will favor: not a larger-than-life liberal to counter the conservative pyrotechnics of Justice Antonin Scalia, but a careful pragmatist with a limited view of the role of courts.

“His nominee will not create the proverbial shock and awe,” said Charles J. Ogletree, a Harvard professor who has known the president since his days as a student.

Mr. Obama believes the court must never get too far ahead of or behind public sentiment, they say. He may have a mandate for change, and Senate confirmation odds in his favor. But he has almost always disappointed those who expected someone in his position — he was Harvard’s first black law review president and one of the few minority members of the University of Chicago’s law faculty — to side consistently with liberals.

Former students and colleagues describe Mr. Obama as a minimalist (skeptical of court-led efforts at social change) and a structuralist (interested in how the law metes out power in society). And more than anything else, he is a pragmatist who urged those around him to be more keenly attuned to the real-life impact of decisions. This may be his distinguishing quality as a legal thinker: an unwillingness to deal in abstraction, a constant desire to know how court decisions affect people’s lives.

“The University of Chicago was and is full of eminent theorizers who wrap up huge areas of the law by applying some magic key,” said David Franklin, a former student. “He didn’t do any of that; he wasn’t interested in high theory at all.”

Though Mr. Obama rarely spoke of his own views, students say they sensed his disdain for formalism, the idea — often espoused by Justices Scalia and Clarence Thomas, but sometimes by liberals as well — that law can be decided independent of the political and social context in which it is applied. To make his point, Mr. Obama, then a state senator, took students with him to Springfield, Ill., the capital, to watch hearings and see him hash out legislation.

And he asked constant questions about consequences of laws: What would happen if a mother’s welfare grant did not increase with the birth of additional children? As a state legislator, how much could he be influenced by a donor’s contribution?

Former students say that Mr. Obama does not particularly prize consistency or broad principle. Adam Bonin arrived in Mr. Obama’s class with the firm belief that drawing districts to ensure minority representation should be illegal. “It struck me as wrong that the legislature should pick and choose what interests should be represented in the legislature,” Mr. Bonin said.

“What I took from the class and the reading materials was the reality that unless these voices are physically present in a legislature, they won’t be heard,” he said. “As long as everyone is grabbing for power, members of racial minority groups ought to do the same.”

But when it came to sentencing laws, Mr. Obama led Mr. Bonin in a more conservative direction than the student had expected. The primary victims of black criminals were fellow blacks — and so minority neighborhoods had an interest in keeping sentencing laws tough, he taught.

Pragmatism has its detractors, and in a confirmation battle, Mr. Obama’s nominee could face charges that he or she does not give enough weight to formal law. But although Mr. Obama is results-oriented, he retained an overall skepticism for what courts can accomplish, said David Strauss, a former colleague at University of Chicago. In Mr. Obama’s due process and voting right classes, he showed students the broad failures of Reconstruction-era amendments that tried to establish equality for blacks.

“He sees the political process as the place that a lot of these large, difficult public policy questions ought to be resolved,” said Richard Pildes, a professor at New York University law school whom Mr. Obama met through their mutual interest in election law.

Even as law review president, Mr. Obama de-emphasized his own views and instead made himself a channel for those of others. His decision making was “about the group sentiment and what the group majority might agree to,” said Nancy McCullough, a fellow editor.

In class and in conversation, Mr. Obama talked about judges all the time, but in heterodox terms that gave no clear sense of whose work he most prized.

“I would imagine that if Barack had a free hand to appoint judges without having to worry about confirmations, about politics, that his idea of a great justice would be someone like a Thurgood Marshall,” said Geoffrey Stone, a former dean of the University of Chicago law school.

Mr. Obama often expressed concern that “democracy could be dangerous,” Mr. Stone said, that the majority can be “unempathetic — that’s a word that Barack has used — about the concerns of outsiders and minorities.”

But when a student asked Mr. Obama to name the circuit judge he would most like to argue in front of, he named Richard Posner, a conservative. Judge Posner was smart enough to know when you were right, Mr. Obama told the class.

And in his seminar discussions, he poked holes in the arguments of Justices Marshall, a liberal, and Thomas, a conservative, alike.

Mr. Obama’s selection of a new justice may challenge him in a way that running a law review and then teaching law never did. Both of those jobs were about cultivating robust debate, about encouraging multiple viewpoints. Now Mr. Obama must settle on a single legal thinker — at least for now.

Thanks to his time in Cambridge and Chicago, he knows the options rather well.

Cass R. Sunstein, Elena Kagan and Diane Pamela Wood, three names likely to be on Mr. Obama’s list, are all former colleagues at University of Chicago. When Mr. Sunstein was married last summer, Mr. Obama sent a long toast to be read at the wedding. Another possible nominee, Pamela S. Karlan, co-wrote an election law textbook that Mr. Obama not only taught from but also contributed comments to when it was in draft form.

In class, Mr. Obama liked to tell students that the Supreme Court was not as far off as it seemed, that it was a dynamic institution that they should not be afraid to challenge and change.

One day in class, Aleeza Strubel told Mr. Obama that she was confounded by a particular decision, a voting rights opinion she simply could not understand. He quizzed her carefully on the court’s logic but finally acknowledged it was rather hard to grasp.

“When you clerk for Justice O’Connor,” Mr. Obama said, jokingly, “you’ll tell her she got it wrong.”