Kagan, 50, taught alongside Obama at the University of Chicago Law School in the 1990s before going on to serve as the dean of the Harvard Law School. An alumna of the Clinton administration, Kagan now supervises all government litigation before the Supreme Court.
Obama will nominate Kagan on Monday, kicking off a confirmation process in the Senate that the White House hopes will go smoothly because of her personal track record with conservatives.
If she is confirmed, the court will have three female justices for the first time. At age 50, she would be the youngest member of the court and the only one without previous judicial experience. She would also be the fourth to have grown up in New York City, after Justices Antonin Scalia, Ruth Bader Ginsburg and Sonia Sotomayor. Kagan grew up in Manhattan and graduated from Hunter College High School.
She would be the third justice in a row to have gone to Princeton as an undergraduate. Justice Samuel A. Alito Jr. and Sotomayor were also Princeton grads. Her appointment would also leave the court without a Protestant for the first time. Currently, six of the justices are Catholics and two are Jewish, as is Kagan.
As dean at Harvard, Kagan developed good relations with conservative professors, students and alumnae despite her progressive credentials.
She once hosted a celebratory dinner for conservative Justice Antonin Scalia when he marked his 20th anniversary on the high court, and another time she drew a standing ovation from members of the Federalist Society during a national convention on campus.
In part because of that track record, some liberal activists had come to consider Kagan one of the more conservative people on Obama's working list of potential nominees.
But advisors to the president characterize her as a progressive who is able to find common ground with legal thinkers across the left-right spectrum.
Obama advisors say that is what the president was looking for: a nominee who might work to keep the court's five conservatives from running roughshod over the four liberals.
Kagan would replace Justice John Paul Stevens, a regular member of the court minority.
The White House is hoping to move Kagan through the Senate hearing process this summer, in time for her to be seated when the court convenes this fall.
Kagan's nomination is not much of a surprise in political circles, where her name has been in play for months. She surfaced as a possible candidate when a Supreme Court position came open last spring, but Obama appointed Sotomayor instead.
When Stevens announced his retirement, Kagan was thought to have the inside track to the nomination. In academia and in politics, she has worked with key players in Obama's circle.
Kagan would be the first high court justice since 1971 with no judicial experience. That year, President Nixon chose Lewis F. Powell and William H. Rehnquist, even though neither had served as a judge.
Despite her years as a law professor, Kagan has managed to avoid taking stands on controversial subjects such as abortion or affirmative action. While at Harvard, however, she joined a lawsuit that contested whether military recruiters could come to the campus to interview graduates. Harvard and several other law schools had a policy against having recruiters for employers who discriminated based on sexual orientation.
When the lawsuit reached the Supreme Court, the justices unanimously upheld the authority of Congress to require equal treatment for military recruiters.
Some conservatives say Senate Republicans should question Kagan closely because she has a limited public record. They cite her 1995 law review article arguing that senators should require nominees to talk about their views on legal controversies. In that article, she said that Senate hearings are a "vapid and hollow charade" where nominees are permitted to "stonewall" and avoid detailing their actual views. Clarence Thomas was confirmed even though his "substantive testimony had become a national laughingstock," she wrote.
"It's especially important that the Senate hold Kagan to the Kagan Standard," said Ed Whelan, president of the Ethics and Public Policy Center. "Among Supreme Court nominees over the last 50 years or more, Kagan may well be the nominee with the least amount of relevant experience."
Carrie Severino, general counsel of the Judicial Confirmation Network, a conservative interest group, said the debate over Kagan will center on her status as an administration insider who was picked, as she charged, to "rubber-stamp" the president's domestic policy agenda if legislation pertaining to healthcare and financial regulatory reform are challenged in court.
Liberals are already lined up behind her.
"She has an excellent chance, and she would be terrific," Harvard law professor Laurence H. Tribe said. "She has a masterful command of so many areas of law. And she's been vetted and recently confirmed. Her writing is not voluminous, which is also a plus."
Her colleagues at Harvard say they don't doubt Kagan is a liberal. Her heart "beats on the left," as professor Charles Fried put it earlier this spring in the New Republic.
She clerked for liberal Judges Abner Mikva and Thurgood Marshall, he points out, arguing that she is unlikely to turn out to be "some kind of crypto-Republican who would shift the court to the right."
She came to Harvard and "determined that it was her job to make the biggest, richest and most famous law school in the world also the best," he wrote, "and that she would do it by recruiting excellent teachers from across the ideological spectrum."
James Oliphant contributed to this report.