And with any shift in the balance of power comes a corresponding shift in White House strategy should Ginsburg choose to step down. The justice has signaled she would like to retire while a Democrat is president. Right now, the party, with the help of two independents, holds 55 Senate seats. After next year, that number could drop to 52 or even lower, perhaps even below a majority. If Ginsburg's hope is to have a true-blue liberal, or a history-making nominee, take her place, she should announce her retirement—and sooner rather than later, to give the president as much time as possible to secure her successor.

Remember, Ginsburg isn't just any justice. She's a trailblazer, the Court's first liberal female jurist, a former ACLU lawyer who has dedicated her career to fighting for feminist and progressive causes. More than most justices, she has built a legacy, through her work before joining the high court and during her 20-year tenure on it.

If the president wants to select a progressive to succeed her, he would be wise to do it when he has maximum leverage in the Senate. The greater the number of Republicans in the chamber, the greater the chance of a filibuster—and that has never been truer than now. While a filibuster has never been used to keep a justice off the Supreme Court, Majority Leader Harry Reid changed the game last month when he invoked the "nuclear option" to eliminate its use for lower-court nominees. His move dramatically raised the possibility the GOP will seek to pick a fight with a high-court choice it deems too left-leaning. "Now would be the time, if that's going to happen," says Christopher Schroeder, a former high-ranking Obama Justice Department official who was involved in selecting judicial nominees.

If the Senate flips and suddenly it's Senator Chuck Grassley of Iowa or some other Republican running the Judiciary Committee, "it gets harder," says Marge Baker, executive vice president of the liberal advocacy group People for the American Way. Even short of an outright filibuster, a chairman can use a variety of procedural tricks to slow-walk a nomination, especially in a presidential-election year.

Ginsburg has to think about the long game. Her retirement could give the president the opportunity to make history by appointing the first Asian-American justice, someone such as Goodwin Liu, a California Supreme Court justice whom Senate Republicans kept from a federal Appeals Court seat in 2011, or California Attorney General Kamala Harris, who is Asian-American and African-American. Either pick would require Obama to burn more of his rapidly dwindling political capital—and either would likely be a harder sell after 2014. Because of that, in a more closely divided Senate after the midterms, the White House may be tempted to turn to a safer option, such as Merrick Garland, a Washington federal Appeals Court judge who prosecuted terrorism cases for the Justice Department.