Moreover, in his selection of finalists, Mr. Obama effectively framed the choice so that he could seemingly take the middle road by picking Ms. Kagan, who correctly or not was viewed as ideologically between Judge Wood on the left and Judge Garland in the center.

Judge Garland was widely seen as the most likely alternative to Ms. Kagan and the one most likely to win easy confirmation. Well respected on both sides of the aisle, he had a number of conservatives publicly calling him the best they could hope for from a Democratic president. Senator Orrin G. Hatch of Utah, a Republican member of the Judiciary Committee, privately made clear to the president that he considered Judge Garland a good choice, according to people briefed on their conversations.

But Mr. Obama ultimately opted to save Judge Garland for when he faces a more hostile Senate and needs a nominee with more Republican support. Democrats expect to lose seats in this fall’s election, so if another Supreme Court seat comes open next year and Mr. Obama has a substantially thinner margin in the Senate than he has today, Judge Garland would be an obvious choice.

As for Ms. Kagan, strategists on both sides anticipate a fight over her confirmation but not necessarily an all-out war. The White House hopes the Senate Judiciary Committee can hold hearings before July 4, but some Congressional aides were skeptical. Either way, Democrats want Ms. Kagan confirmed by the August recess so she can join the court for the start of its new term in October.

A New Yorker who grew up in Manhattan, Ms. Kagan earned degrees from Princeton, Oxford and Harvard Law School, worked briefly in private practice, clerked for Justice Thurgood Marshall, served as a Senate staff member and worked as a White House lawyer and domestic policy aide under President Bill Clinton. She was nominated for an appeals court judgeship in 1999, but the Senate never voted on her nomination.

She has been a trailblazer along the way, not only as the first woman to run Harvard Law School but also as the first woman to serve as solicitor general. Her inexperience as a judge makes her a rarity in modern times, but until the 1970s many Supreme Court justices came from outside the judiciary, including senators, governors, cabinet secretaries and even a former president.

If the Senate confirms Ms. Kagan, who is Jewish, the Supreme Court for the first time will have no Protestant members. In that case, the court would be composed of six justices who are Catholic and three who are Jewish. It also would mean that every member of the court had studied law at Harvard or Yale.

Like her former boss, Justice Marshall, who was the last solicitor general to go directly to the Supreme Court, Ms. Kagan may be forced to recuse herself during her early time on the bench because of her participation in a number of cases coming before the justices. Tom Goldstein, publisher of ScotusBlog, a Web site that follows the court, estimated that she would have to sit out on 13 to 15 matters. Mr. Whelan argued that it would be significantly more than that.