None have shown any interest in stepping down, though Randall L. Kennedy, a liberal Harvard Law professor, argued last year that Justices Ginsburg and Breyer should quit so Mr. Obama could name younger like-minded replacements. Professor Kennedy presented his argument in an article published in The New Republic under the headline “The Case for Early Retirement.”

“Both are unlikely to be able to outlast a two-term Republican presidential administration,” Professor Kennedy wrote, adding, “What’s more, both are, well, old.”

It was a provocative article; in an interview, Professor Kennedy said the suggestion that a justice should retire for purely political reasons was “viewed as somewhat unseemly” by many of his colleagues. And those close to Justice Ginsburg say that while she may appear frail, she is in fact in good health.

“Justices have a conflicting set of obligations,” said Geoffrey R. Stone, a constitutional law professor at the University of Chicago, where Mr. Obama once taught. “On the one hand, they have an obligation to serve their terms as long as they feel it’s in the interest of the nation, and as long as they feel they can do the job well. But they have a conflicting desire, which is to perpetuate their view on the court. It’s a political and personal judgment which they have to make for themselves.”

Justices leave for a variety of reasons. Sandra Day O’Connor, for instance, left the court at 75 to take care of her husband. Professor Kennedy insists it was “not accidental” that, having been appointed by Ronald Reagan, a Republican, she resigned while George W. Bush was president.

Her announcement in July 2005 caught official Washington by surprise; many had expected the chief justice, William H. Rehnquist, who was being treated for thyroid cancer, to step down. But Justice Rehnquist had privately told Justice O’Connor that he had no intention of quitting. Two months after her announcement, he was dead, and Justice O’Connor, to avoid leaving a second vacancy, agreed to stay on the court until her replacement was confirmed.

The court’s most recent retirees, Justice David H. Souter and Justice Stevens, were appointed by Republicans, but as the court shifted right, they moved left. Justice Souter, who retired at 69, made it clear that he disliked Washington and wanted to move back home to New Hampshire. Both he and Justice Stevens, by then the leader of the court’s liberal wing, stayed on until Mr. Obama became president.