“So anybody who thinks that if I step down, Obama could appoint someone like me, they’re misguided,” she said.

Artemus Ward, the author of “Deciding to Leave: The Politics of Retirement From the United States Supreme Court,” said Justice Kennedy found himself at a crucial crossroads. If he wants to resign under a Republican president in the first half of a presidential term, he must act.

“It’s now or never,” said Mr. Ward, a political scientist at Northern Illinois University. “It’s either this year or you wait until the next election.”

Party loyalty is likely to overcome more subtle concerns about judicial legacy, Mr. Ward said. Justice Kennedy holds the crucial vote in many closely divided cases, and he has been drifting to the left. He has cast conservative votes in cases on campaign finance and gun rights but has lately voted with the court’s liberal wing on gay rights, abortion and affirmative action.

But Mr. Ward said Justice Kennedy is likely to emulate Justice Byron R. White, who drifted to the right after his appointment by President John F. Kennedy, a Democrat. Even so, Justice White said, it was fitting to retire under a Democratic president because he had been appointed by one. He stepped down not long after President Bill Clinton was elected and was replaced by Justice Ginsburg, whose voting record has been considerably more liberal.

A new study on departures from the Supreme Court attempted to refine the conventional factors by considering political science data on justices’ voting patterns, involuntary departures and missed opportunities. It concluded that justices have not been particularly successful at ensuring that they would be replaced by like-minded successors.

Justice Kennedy is in a ticklish spot, said the study’s author, Christine Kexel Chabot, who teaches at Loyola University Chicago School of Law. He is, she wrote, a moderate conservative, and there is no reason to think a president of either party would replace him with someone similar.