Kagan, 50, is considered one of the finest legal scholars in the country, dazzling both fellow liberal and conservative friends with her intellectual and analytical prowess but also her ability to find consensus among ideological opposites.
"She's a solid, hard working, intelligent, really smart lawyer, who's had an extraordinary amount of experience in the law even though she hasn't been a judge," said Greg Craig, former White House counsel, on "Good Morning America." "Politically, I think she's also as mainstream as they can get."
Obama, who called Kagan last night with his final decision, is also said to be impressed with her trail-blazing career as the first woman to head Harvard Law School and the first woman confirmed as solicitor general. Kagan's background as the daughter of a tenant lawyer and a teacher and her never having served as a judge were also important factors in her nomination, sources say.
During her confirmation for solicitor general, Kagan won the support of Democrats and nine Republicans in the Senate, including two conservatives on the Judiciary Committee: Sens. Jon Kyl and Orrin Hatch. In the weeks ahead, Democrats will likely make the case the case that if she's good enough to represent the U.S. before the Supreme Court, she's good enough to be on the court.
But some conservatives are expected to put up a fight, calling Kagan "inexperienced" and divisive in reaction to her nomination.
"Kagan has been nominated with no judicial experience, a mere two years of private law experience, and only a year as Solicitor General of the United States," wrote David McIntosh, co-founder of the conservative Federalist Society. "She is one of the most inexperienced nominees to the U.S. Supreme Court in recent memory."
"She's been extremely guarded about her views, with the exception of gay rights, where she has been vehement in opposing federal laws she doesn't like and has worked as Solicitor General to undermine those laws," said Ed Whelan, president of the conservative Ethics and Public Policy Center.
Still, Kagan supporters say they don't see the criticism as a real threat to her confirmation.
"You don't have to have judicial experience to be a highly qualified candidate," said Craig. "She is her own person -- an independent force of nature."
William Rehnquist and Lewis Powell, both appointed by President Richard Nixon in 1972, were the last two justices to sit on the high court without any prior judicial experience.
Who is Elena Kagan?
Human rights advocates have been furious that as solicitor general Kagan has supported some of the same policies held during the Bush administration, including the argument against extending habeas rights to detainees held in Bagram Air Force base.
But experts caution about reading too much into the positions Kagan has taken while serving as solicitor general.
"Part of the reason why it is difficult to look at the solicitor general for the court is because one can't ever really tell, unless they were in the room, whether she is shaping policy, or merely enforcing it," said law professor Stephen I. Vladeck of American University Washington College of Law. "It's unfair to reach any conclusions based purely on external, public indicia."
Obama has made clear his desire to hold confirmation hearings by early July. In her last appearance before Congress, Kagan was a lively witness, at times charming conservative senators. But a confirmation hearing for a lifetime appointment to the court is sure to entail more pointed questions and a closer review of her record.
Kagan may also have to explain comments she made in a 1995 book review on Senate confirmation fights: "When the Senate ceases to engage nominees in meaningful discussion of legal issues, the confirmation process takes on an air of vacuity and farce, and the Senate becomes incapable of either properly evaluating nominees or appropriately educating the public."
She will also be asked more questions about decisions she made while working in the Clinton White House. The administration has yet to release any documents from that time period.
During her confirmation hearings for solicitor general, she was asked about several memos that she prepared in 1987 for her then-boss Justice Thurgood Marshall. Conservatives believe the memos on criminal procedure reveal Kagan to be left of the political spectrum.
At her hearing, Kagan downplayed the effect of the memos, saying, "You know, I was a 27-year-old pipsqueak, and I was working for an 80-year-old giant in the law, and a person who, let us be frank, had very strong jurisprudential and legal views."
Sen. Arlen Specter of Pennsylvania asked her about one memo involving the Adolescent Family Life Act, which authorized federal funds for religious organizations designed to discourage teen pregnancy. Specter, then a Republican who has since become a Democrat, quoted Kagan's memo: "It would be difficult for any religious organization to participate in such projects without injecting some kind of religious teaching."
At the hearing, Kagan admitted she had only recently seen the memo again after 20 years. "And I looked at it and I thought that is the dumbest thing I've ever heard," she said. Specter laughed.
ABC News' Jake Tapper, Jonathan Karl and Devin Dwyer contributed to this report.