“Having concluded that it would be in the best interests of the Court to have my successor appointed and confirmed well in advance of the commencement of the Court’s next Term, I shall retire from regular active service as an Associate Justice,” Justice John Paul Stevens has written in a letter delivered today to President Obama.

Justice Stevens, the senior member of what is now identified as the liberal wing of the Supreme Court (although he came to the court as and in many senses remained an old-school moderate Republican), has decided to retire “effective the next day after the Court rises for the summer recess this year.”

What this means is that President Obama will have an opportunity to appoint a second justice, following his nomination last year of Justice Sonia Sotomayor. (He may get a third pick soon, as well, since there is much speculation about the prospect that another member of the court’s liberal wing, ailing 76-year-old Justice Ruth Bader Ginsberg, might soon step down.)

Just as Sotomayor’s selection did not change the ideological balance on the court, as she replaced liberal Justice David Souter, Obama’s choice to fill the Stevens seat will not tip the conservative-leaning high court to the left.

But it will almost certainly open up a brutal battle in an election year over not just the court but all the social, economic and balance of power issues that make the fights over court picks such high-profile struggles. Justice Stevens may have joined the court as a Republican appointee (he was nominated by President Gerald Ford in the aftermath of Watergate) but he served for the most part as a champion of civil liberties and an expansive interpretation of the Constitution.

Progressives will hope for a replacement who is not merely as liberal but as savvy in the ways of the court as was Justice Stevens. Conservatives will look for any opening to make a court that was moved well to the right by former President Bush’s appointments can be moved in an even more reactionary direction.

Obama, a lawyer and former constitutional law professor, has already outlined his values when it comes to court picks, saying when Suter left that he would look for a nominee with the “quality of empathy, of understanding and identifying with people’s hopes and struggles, as an essential ingredient for arriving as just decisions and outcomes.”

Most of the names that have been floated as potential nominees to success Justice Stevens fit that standard: Solicitor General Elena Kagan, Federal Judge Diane Wood, Homeland Security Secretary Janet Napolitano, Michigan Governor Jennifer Granholm, Minnesota Senator Amy Klobucher and former Georgia Supreme Court Chief Justice Leah Ward Sears. The name of Cass Sunstein, the head of the White House Office of Information and Regulatory Affairs, is also in play.

In replacing Justice Stevens, Obama will look for a progressive. But he must also look for a nominee who will have have the maturity, the boldness and the courage that the retiring justice has displayed as he has struggled to defend the Constitution and the rule of law in turbulent times.

Justice Stevens will, of course, he recalled as a wise and effective advocate of moves by the court to enforce a strict separation of church and state, defend affirmative action, limit the use of the death penalty and expand access to reproductive rights. As National Organization for Women president Terry O’Neill said: “The Supreme Court will lose an influential member, and women will lose a real champion when Justice John Paul Stevens retires at the end of this session…. Since 1975, Justice Stevens has played a critical role in a number of key Supreme Court decisions. Stevens was a consistent vote in upholding Roe v. Wade, and he has supported affirmative action, LGBT rights and other principles of social justice.”

While all of what O’Neill says is true, Justice Stevens contributed something else to the court.

He recognized places where the justices ought not intervene.

His stinging dissent in the case of Bush v. Gore, where he opposed the court majority’s lawless blocking of the recount of Florida ballots following the 2000 presidential vote, was as historic as it was chillingly prescient. Justice Stevens argued with passion and precision about the folly of the judicial activism of conservative justices who were determined to clear the way for George Bush to assume the presidency prior to the settlement of a disputed election.

In this regard, and despite his reputation as a liberal, it was Justice Stevens — not the activist majority — who upheld the principle of judicial conservatism and respect for the original intent of the Constitution.

He also respected the court to a far greater extent than those who used it as a vehicle to decide an election dispute that should have been resolved at the state level.

“Although we may never know with complete certainty the identity of the winner of this year’s presidential election, the identity of the loser is perfectly clear,” Stevens wrote. “It is the nation’s confidence in the judge as the impartial guardian of the rule of law.”

Justice Stevens was right. But, while the court’s reputation suffered, his did not.

The 89-year-old justice will finish his tenure with the reputation as the court’s boldest and most courageous champion of the Constitution he swore to uphold.

Obama must appoint an equally bold and unapologetic champion to fill the void created by the departure of this unapologetic defender of the founding document as the living and liberal charter that the founders intended.