Justice Ruth Bader Ginsburg was widely, and deservedly, criticized for her refusal to retire from the Supreme Court at a time when a Democratic president could have chosen her replacement.

Justice Stephen Breyer is making a similar and arguably even more egregious mistake.

The evident indifference on the part of Democrats regarding the failure of Justice Breyer, 82, to announce his retirement is apparently a product of the assumption that he will do so at some point during the current Congress and that therefore whether he does so anytime soon is not particularly important.

This is a grave mistake.

Consider that because of the extremely thin nature of their Democratic Senate control, the shift of a single seat from the Democrats to the Republicans or even one vacancy in the 50 seats now controlled by the Democratic caucus would probably result in the swift reinstallation of Mitch McConnell as the majority leader.

What are the odds that something like this — a senator’s death, disabling health crisis or departure from office for other reasons — will happen sometime in this Congress’s remaining 22 months?

Alarmingly for Democrats, if history is any guide, the odds are quite high. Since the end of World War II, 27 of the 38 Congresses have featured a change in the party composition of the Senate during a session.

The probability that such a shift may occur during this particular Congress may well be even higher than that. At the moment, no fewer than six Democratic senators over the age of 70 represent states where a Republican governor would be free to replace them with a Republican, should a vacancy occur.

Five other Democratic senators represent states for which a vacancy would go unfilled for months, until a special election to fill the seat was held — which would hand the G.O.P. control of the Senate at least until that election and likely for the rest of the current Congress if a Republican wins that contest. (In the case of Wisconsin, such a vacancy might not be filled until 2023.)

All things considered, the odds that Democrats will lose control of the Senate in the next 22 months are probably close to a coin flip.

Under the circumstances, for Democrats to run the very real risk that they would be unable to replace Justice Breyer is unacceptable. Of course, the only person who is in a position to ensure that this does not happen is Justice Breyer himself.

It is true that, under normal circumstances, a Supreme Court justice planning to retire generally waits until the end of a court term to do so. But these are not normal circumstances.

Nothing illustrates the anti-democratic dysfunction of our political system more clearly than the current makeup of the Supreme Court. Two-thirds of the sitting justices were nominated by Republican presidents, even though Republican presidential candidates have lost the popular vote in seven of the nine elections, which determined who nominated these justices.

And these justices were confirmed by a Senate that has become skewed so radically in favor of electing Republicans that the 50 senators who caucus with the Democrats represent about 41.5 million more Americans than the 50 Republican senators do.

Under the circumstances, it would be a travesty if the Supreme Court seat occupied by Justice Breyer was not filled by a replacement chosen by Democrats.

He should announce his retirement immediately, effective upon the confirmation of his successor. For him to continue to make the same gamble that Justice Ginsburg made and lost runs the risk of tainting his legacy as a justice and has the potential to be an anti-democratic disaster for the nation as a whole.

Paul F. Campos is a law professor at the University of Colorado, Boulder, and writes about law and politics at Lawyers, Guns & Money.

The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.

Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram.