Though not widely known yet, both traveled along the rarefied network of the powerful in 2005, and since then, both have taken a fast track upward, Obama winning the presidency and Kagan winning confirmation as the solicitor general.
Monday, by picking Kagan for the lifetime job as Supreme Court justice, Obama, in many respects, chose someone who appears to be much like him.
They both have reputations as being intellectually superior, had limited experience for their current jobs, and are seen as liberal but pragmatic Democrats who can be unpredictable, activists on both sides say.
Both are trailblazers - as the first black president and the first woman Harvard Law dean and solicitor general - who also are known as consensus builders who reached out to conservatives at Harvard.
But the question Senate Democrats and Republicans will be asking is: How close are their views of the Constitution and judicial philosophy?
Right now, no one knows, making Kagan, experts said, a "stealth nominee" whom senators will grill at Senate Judiciary Committee hearings this summer.
But she never served as a judge who produced a stack of rulings and she spent just a few years as a lawyer. Her academic writings reveal little. A year ago, Kagan answered so few questions in hearings to become solicitor general that then-GOP Sen. Arlen Specter (D-Pa.) voted against her. She was confirmed in a 61-31 vote.
Conservative activists Ed Whelan and Curt Levey said earlier this year Kagan did not rank among the more liberal possible picks, a rating liberals also offered.
Sen. Jeff Sessions (R-Ala.), who will lead GOP questioning of Kagan, said he doesn't know enough to "classify" her views.
Sen. Charles Schumer (D-N.Y.) called her a moderate.
Kagan's political experience, according to her Senate questionnaire last year, came in the Democrats' centrist wing.
As Clinton's domestic policy adviser, she helped fashion the welfare reform that led two officials to quit in protest. But she also worked on bills to limit tobacco use and hate crimes.
Monday, the left and right flagged key questions.
The left cited executive authority and anti-terror policy.
"Kagan's record indicates a troubling support for expanding presidential powers," which resulted in torture, illegal detention and profiling, said the Center for Constitutional Rights.
Backers say Kagan spoke out against torture and profiling. Yet, as Harvard dean, she sent one letter to Congress against a bill to bar court hearings of petitions from Guantánamo detainees. Obama also has drawn criticism for not reversing more Bush-era anti-terror policies.
The right and some Republicans cited military recruitment.
Kagan "spit in the eye of America's armed forces," said the Committee for Justice, by banning recruiters from Harvard Law in her opposition to the military's "don't ask, don't tell" policy on gays. Kagan joined two briefs asking the court to overturn that policy, but let recruiters return to avoid losing federal funds. Obama also seeks to change the policy, but has let the Pentagon set the pace.