Sonia Sotomayor will turn 70 this June. If she retires this year, Biden will nominate a young1 and reliably liberal judge to replace her. Republicans do not control the Senate floor and cannot force the seat to be held open like they did when Scalia died. Confirmation of the new justice will be a slam dunk, and liberals will have successfully shored up one of their seats on the court — playing the kind of defense that is smart and prudent when your only hope of controlling the court again relies on both the timing of the deaths or retirements of conservative judges, plus not losing your grip on the three seats you already hold.

But if Sotomayor does not retire this year, we don’t know when she will next be able to retire with a likely liberal replacement. It’s possible that Democrats will retain the presidency and the Senate at this year's elections, in which case the insurance created by a Sotomayor retirement won’t have been necessary. But if Democrats lose the presidency or the Senate this fall (or both) she’ll need to stay on the court until the party once again controls both. That could be just a few years, or it could be a while — for example, Democrats have previously had to wait 14 years from 1995 to 2009, and 12 years from 1981 to 1993.2 In other words, if Sotomayor doesn’t retire this year, she’ll be making a bet that she will remain fit to serve through age 82 or 84 — and she’ll be taking the whole Democratic Party coalition along with her in making that high-stakes bet.