News reports that the 89-year-old Supreme Court Justice John Paul Stevens is contemplating retirement have launched a flurry. Rumors began circulating last September when court watchers noticed that he hadn't hired the usual number of clerks for the 2010 term. Ever since, the legal community has been expecting an imminent announcement. But in an interview published last month, Stevens expressed uncertainty about whether he would indeed step down before the next term. “I still have my options open,” he said.

Some people are urging Stevens to stay on. Over the weekend, Senator Arlen Specter said he thought Stevens shouldn't retire now given the dynamics of Washington politics. In today's overheated, partisan atmosphere, the person President Obama picks to succeed Stevens could trigger a filibuster. Congress, Specter suggested, can't afford to get bogged down on a judicial nominee.

Stevens has had a remarkable career and will long be remembered for his influence on a changing court. He should preserve his legacy by retiring now.

It's time, however, for Stevens to step down. Washington's partisanship isn't going to dry up any time soon. Republicans seem certain to gain seats in November, making the confirmation process only more difficult for an Obama nominee. Moreover, the court and the country can't afford to have Stevens stay on too long. Everyone who knows Justice Stevens insists that he’s in fine mental and physical health. But there are reasons to be worried. In January, when he read one of his opinions from the bench, his voice was shaky, he stumbled over easy words, and appeared to lose his place. Maybe he was just having a bad day. But as he moves into his nineties, bad days like that are only going to become more common.

It’s easy to understand Stevens’ reluctance to leave. Supreme Court justices are among the most powerful people in the country. They have terrific jobs where they control their own workload and get to weigh in on most of the major issues of the day with a weightier voice than any senator or congressional representative. Nonetheless, the time has come for Stevens to step down.

• Eric Alterman: Will Obama Screw the Left with His SCOTUS Nominee?When it comes to retirement, Supreme Court justices are like sports stars: Too many of them refuse to admit they should move on. But unlike athletes, justices, who have life-tenure, don’t need a desperate team to keep paying them. The marketplace eventually forces even the greatest athletes to quit. But justices often stay on long after their capabilities have diminished.

Historian David Garrow, who has studied the retirement patterns of justices, says that “the history of the court is replete with instances of justices casting decisive votes” even “when their colleagues and/or families had serious doubts about their mental capacities.” The man who occupied Stevens’ seat before him, William O. Douglas, is a good example. He served longer than any other justice in American history, staying well past the day he should have retired. He refused to go despite suffering a severe stroke. In response, the other justices conspired to push off any case in which his vote might be decisive. When Douglas finally recognized the problem and retired, his mind was so far gone that he continued to show up at the Supreme Court demanding to be allowed into his old office. He was, he said in his dementia, still a member of the court.

The dilemma is as old as the court itself. Appointed chief justice by George Washington, John Rutledge was one of the signers of both the Declaration of Independence and the Constitution. But after the death of his wife, he suffered debilitating depression and tried to commit suicide while serving as the nation’s top jurist. Another justice, Henry Baldwin, was hospitalized for “incurable lunacy” soon after he joined the court. But despite being described by one of his colleagues as “partially deranged at all times,” Baldwin remained on the court until his death more than a decade later.

Part of the problem is that justices who should retire because they are losing their mental acuity don’t have, well, the mental acuity to know they should retire. So they stay on, making vital decisions about the health and welfare of the nation. If, however, they can’t make the right decision about retirement, how can we trust them to make the right decision about, say, the constitutionality of health-care reform?

In recent years, legal scholars and political scientists have suggested ways to resolve this age-old old-age problem. One proposal is to get rid of life tenure and adopt term limits for justices. Each justice could be given, for instance, a non-renewable 15-year term of service. This would provide them the necessary guarantee of judicial independence and afford them sufficient time to gain the expertise and wisdom that comes from years of service. Other proposals include a mandatory retirement age.

The problem with both these solutions is that they depend on amending the Constitution. In our overheated, highly polarized political atmosphere, who wants to open up the Constitution for major changes?

Still, the situation is only getting worse. In the old days, a justice incapable of doing his work put an extra burden on the other justices. They would have to rewrite his opinions and shoulder his share of the workload. This created a good incentive for the other justices to lobby the infirm one to step down. No one but another Supreme Court justice will have the kind of influence necessary to push a justice to retire.

Today, however, all the justices rely heavily on their numerous law clerks. If a justice can’t handle his load, his clerks can fill the void and the chambers will still be productive. The other justices won’t be put upon, which reduces the likelihood that they’ll get involved. But then we’re left with decisions being made by a less-than-capable justice—or, just as bad, by his twenty-something law clerks.

One reason justices stay on is that even a mentally diminished jurist knows who the president is. Some won’t step down and give the opposing political party a chance to name his successor. Civil-rights icon Thurgood Marshall refused to retire for many years because Ronald Reagan was in the White House. Eventually, Marshall was in such bad health that he had to resign during the administration of George H.W. Bush, who appointed Clarence Thomas as a successor. No doubt Marshall wished he had stayed put.

This concern is clearly part of Stevens’ thinking. Although originally appointed to the court by Republican Gerald Ford, Stevens is now one of the most liberal members of the court. Some say he is a prime example of the “Greenhouse Effect”—the idea that justices turn to the left to appeal to a liberal press, like former longtime New York Times reporter Linda Greenhouse. Stevens insists he hasn’t changed, the court has. With the appointment of increasingly more conservative justices, his moderate positions seem more to the left. Either way, it’s clear Stevens wants his successor to be chosen by a Democrat. He’s said that he will definitely retire before the end of Barack Obama’s term in office.

Stevens has had a remarkable career and will long be remembered for his influence on a changing court. He should preserve his legacy by retiring now.

Adam Winkler is a constitutional law professor at UCLA.