If Mr. Obama chooses another liberal, it will not alter the fundamental ideological balance on the court. Nonetheless, the loss of Justice Stevens’s personal charm, canny tactics and institutional memory can only leave the court’s more liberal wing in a weakened position.

Justice Stevens, who will turn 90 on April 20, is the longest-serving member of the current Supreme Court by more than a decade. He became the senior justice in 1994 with the retirement of Justice Harry A. Blackmun.

That position matters. When the chief justice is not in the majority, the senior justice in the majority is given the power to assign the majority opinion. For the last decade and a half, that justice has almost always been Justice Stevens, and he has used that power with patience and skill to forge and maintain alliances in major liberal victories, often locking in Justice Anthony M. Kennedy’s swing vote by assigning the opinion to him.

Justice Stevens has played a significant and often behind-the-scenes role in cases involving affirmative action, abortion rights and executive power. He grew disillusioned with the death penalty over the years, announcing in 2008 his conclusion that it violated the Eighth Amendment. But he went on to say that his conclusion did not justify “a refusal to respect precedents that remain a part of our law.”

Image President Gerald R. Ford in 1975 with Chief Justice Warren E. Burger and a new justice, John Paul Stevens, who would go on to become the longest-serving justice by more than a decade. Credit... George Tames/The New York Times

And he wrote major dissents in two of the court’s most hard-fought recent 5-to-4 decisions, one ruling that the Second Amendment protects an individual right to own guns, the other that corporations may spend freely in candidate elections. In that second case, Citizens United, Justice Stevens for the first time showed his age on the bench, stumbling a bit as he read a 20-minute dissent.

But for all his influence, he was never well known to the public. When Americans are asked to name members of the Supreme Court in public opinion surveys, his name is routinely the least likely to be mentioned.