A great deal turns on who picks Ginsburg’s successor. There are, for example, four likely votes to overturn Roe vs. Wade on the current court: Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Clarence Thomas, and Samuel A. Alito Jr. If a Republican president selects Ginsburg’s replacement, that justice easily could be the fifth vote needed to allow the government to prohibit all abortions. On many cases — including ones involving environmental law, healthcare, gay marriage, the death penalty and the rights of those in Guantanamo — the four liberal justices have joined with Justice Anthony M. Kennedy for a progressive result.

But if a conservative had been occupying Ginsburg’s seat when the court heard those cases, the rulings might well have been very different, and if a conservative takes her seat when she leaves, they might not survive.

There likely will be many calls, publicly and privately, for Justice Ginsburg to resign before President Obama leaves the White House to prevent the risk of a Republican being able to appoint her successor. But simply leaving before the next election isn’t enough. If Ginsburg waits until 2016 to announce her retirement, there is a real chance that Republicans would delay the confirmation process to block an outgoing president from being able to fill a vacancy on the Supreme Court. In fact, the process for confirming nominees for judicial vacancies usually largely shuts down the summer before a presidential election.