Ruth Bader Ginsburg, by all accounts, has been an excellent Supreme Court justice and is still, at 80, able to do the job as well as ever, maybe better. But she’s jeopardizing everything she believes in by staying on the court, and if the interview she gave to Adam Liptak of the New York Times is any indication, the problem isn’t selfishness; it’s just that she really doesn’t understand the way nomination politics works these days:

Justice Ginsburg said her retirement calculations would center on her health and not on who would appoint her successor, even if that new justice could tilt the balance of the court and overturn some of the landmark women’s rights decisions that are a large part of her legacy.
“I don’t see that my majority opinions are going to be undone,” she said. “I do hope that some of my dissents will one day be the law.”

It’s very simple: If nothing else changes and a Republican appoints her successor, there will be a 5-4 majority against a lot of the things that Ginsburg cares about. There’s not going to be any more David Souter results, in which a Republican accidentally appoints a reliably liberal justice. Or, for that matter, any case-by-case pragmatists, new versions of Sandra Day O’Connor who might be inclined toward conservative positions but flexible on specific cases. The next choices by a Republican president will be very much like the last few — perfectly reliable, and fairly partisan, conservatives. Who, as Ginsburg recognizes, are about as “activist” as any justices have ever been about overturning laws and precedents.

Not only that, but there’s every chance that Justice Anthony Kennedy, 77 now, will retire during the next Republican presidency. The result, should Ginsburg also leave, would be five reliable votes for radical conservative results.

It’s not just Ginsburg; Justice Stephen Breyer is 75. If Republicans win the White House in 2016 and hold it for three terms (and, yes, that’s a perfectly plausible scenario), would he be able to stay on the bench until 2029?

Ginsburg and Breyer may not fully realize what’s happened to judicial confirmations since their own relatively easy Senate votes. Should Republicans win the Senate in 2014 — again, quite possible — confirmation of an Obama nominee would surely be far more difficult. Indeed,  if Ginsburg and Breyer resigned, it’s not far-fetched to imagine Chuck Grassley declaring a new “principle” that seven justices were quite enough, really; after all, the justices’ workload is even lighter than that of judges on the U.S. Court of Appeals for the D.C. Circuit!

It’s possible that Ginsburg and Breyer don’t care about any of that and simply want to continue in office for purely personal reasons. But if they believe that Democrats have a lock on the White House, or that Republicans would replace them with moderates, or that a Republican Senate would confirm someone similar to what they were in the 1990s, they’re just plain wrong, and they’re risking a lot on poor political analysis.

It’s obviously asking a lot, but both Ginsburg and Breyer have had two decades on the nation’s highest court. If they care about the principles they’ve fought for in those two decades, the best thing they could do to continue that fight is to leave the court. As soon as possible.