As President Barack Obama considers who to nominate for an upcoming vacancy on the U.S. Supreme Court, a senior advisor tells the Associated Press that he is looking to choose the person best able to assume, sooner or later, the influential leadership role of retiring Justice John Paul Stevens.

"You can't replace someone's 34 years on the court, but you are mindful of the fact that he was a leader on the court, and you want someone who can provide that kind of spark and leadership—if not immediately, then over time," says David Axelrod. "So he's thinking about that."

While Axelrod says the president hasn't yet made a decision, the AP says he seems to be considering a short list of four candidates: U.S. Solicitor General Elena Kagan, Judge Merrick Garland of the federal appeals court in Washington, D.C.; Judge Sidney Thomas of the 9th U.S. Circuit Court of Appeals; and Judge Diane Wood of the 7th U.S. Circuit Court of Appeals.

Observers anticipate that Obama's selection could be announced at any time.