No single appointment has the potential to define a presidency more than the naming a Supreme Court justice. But for all its headline-grabbing qualities, the decision rests on a fairly fluid set of political considerations: a prospective candidates’ personal qualities, past experience and political instincts.

President Obama surely is prepared to undertake such an examination.

In spite of the economic crisis, two wars and an almost unimaginably crowded domestic agenda, the president knew long before he took office that he’d probably have to fill a vacancy on the high court early in his first term.

And at a time when he has abundant political capital, experts say Obama views the retirement of Justice David H. Souter as an opportunity to select a judicial activist who’s committed to upholding decisions revered by the political left.

Though such an appointment will almost certainly lead to acrimonious hearings and a potentially lengthy confirmation fight, it also promises to strengthen Obama’s political base while challenging an already weakened Senate Republican caucus to try to scuttle the nomination.

Senate Democrats considered the strategy when they were the minority party during the confirmation hearings of John G. Roberts Jr. and Samuel A. Alito, but concluded it was both tactically difficult and politically dicey, because of the way it could portray them as partisan and obstructionist.

“Republicans will do their best to talk up the issues and at the end of the day get 30 or 35 votes against, but nobody’s actually going to try to block the nominee,” predicted Richard Samp, chief counsel of the conservative Washington Legal Foundation.

Obama spoke with Souter on Friday afternoon and afterward interrupted the daily White House press briefing to announce the justice’s retirement and praise Souter’s “good sense of humor . . . integrity, equanimity and compassion.”

“I will seek someone who understands that justice isn’t about some abstract legal theory or footnote in a casebook. It is also about how our laws affect the daily realities of people’s lives,” said Obama, a former constitutional law professor. “I view that quality of empathy, of understanding and identifying with people’s hopes and struggles, as an essential ingredient for arriving at just decisions and outcomes.”

White House press secretary Robert Gibbs said one recent decision that didn’t reflect such an understanding came in the 2007 case of Lily Ledbetter, in which the Supreme Court ruled against Ledbetter’s pay disparity claim because the statute of limitations had run out by the time she discovered the discrepancy.

In January, the first bill Obama signed into law lifted the cap on compensatory and punitive damages that women may be awarded in wage discrimination cases.

As a candidate, Obama said he would not appoint a nominee who does not subscribe to the right of privacy established in Roe v. Wade, the 1973 Supreme Court case that found the Constitution protects access to abortions, and Gibbs said Friday the president would abide by that pledge. However, Obama did not make judicial nominations a prominent part of his campaign.

Sarah Binder, a senior fellow in governance studies at the Brookings Institution, said Obama will likely try to select a justice who is reliably left of center but not a liberal icon.