Associate Justice Ruth Bader Ginsburg in her Supreme Court chambers in Washington, two weeks ago. Ginsburg says the Supreme Court won't duck the issue of same-sex marriage the next time a case comes to the court. (AP Photo/Cliff Owen)

To a group of Democratic donors at an event on Martha's Vineyard, President Obama made a somber prediction. A faction of Republicans is worried only about holding on to power, he said, according to the pool report. "And that’s why I need a Democratic Senate -- not to mention the fact that we’re going to have Supreme Court appointments, and there are going to be a whole host of issues that many people here care about that are going to be determined by whether or not Democrats retain the Senate."

It's not clear what those appointments might be. There had been speculation for some time that Justice Ruth Bader Ginsburg might step down, but she tamped that down in an interview with Yahoo News a few weeks ago. "I am still here and likely to remain for a while," she said. Ginsburg, at 81, is the oldest member of the court. If she's not going anywhere, who is?

Very possibly no one. It is clear why Obama said this, of course. He's talking to a room full of people who'd paid at least $15,000 to hear him speak, and trying to make his best case for why they should keep their wallets open. The threat of imminent Supreme Court vacancies was used to great effect during the 2012 race -- a reminder that should John Roberts or Antonin Scalia suddenly decide they'd like to sail around the world, Democrats might prefer President Romney not be perusing binders full of lawyers. This is the 2014 version of that: Do you want Sen. Mitch McConnell's (R-Ky.) gavel hovering over the Senate's vote on an appointee? And there's additional subtext. Maybe, some think, Ginsburg will leave while there's a Democratic president, to save a liberal slot on the bench.

The recent history of Supreme Court retirements, though, suggests that politics don't come into play from the standpoint of the justices. The polarization of American politics as a recent phenomenon is well-documented, of course, but there appears to be no example in the last 50 years of a justice leaving his or her position in order to ensure a like-minded replacement.

We looked at the retirements of the last 10 justices to figure out how they related to the party of the president and the composition of the Senate. In only one case did a justice retire right before the Senate changed hands, while a politically-similar president was serving out his term. In most cases, though, the retirements came a year or more before a midterm election -- often just after a presidential election, not before.

This graphic uses retirement dates from and political leaning of the justices (the colored box next to their names) from this academic study.

Look at Chief Justice Warren Burger, third from the top. A conservative justice, he announced his retirement in September 1986, shortly before the midterms. Sure enough, the Senate flipped Democratic in the next Congress, just in time for the final two years of Ronald Reagan's presidency.

But Burger didn't leave because he wanted to make sure Reagan could appoint another Republican. He left, according to his own statements at the time, to chair the commission celebrating the bicentennial of the Constitution in 1987. Reagan appointed associate justice William Rehnquist to succeed him as Chief Justice, and Rehnquist was followed on the court by Scalia. The composition of the Senate didn't make any difference; Scalia, no known as perhaps the most conservative justice, was approved by a 98-0 vote.

What Obama is suggesting is a situation more like Lewis Powell or Thurgood Marshall, both of whom left between the midterms and the presidential race. But neither example maps to what Obama threatens: Marshall left while the president was a Republican, and Powell left while the Senate was firmly in Democratic hands.

Ginsburg (or anyone else) could leave the Court before 2016. Obama and his party would love to have a Democratic-controlled Senate there to choose her replacement. But there's no recent history of justices leaving simply to be acquiescent to partisan needs, no matter how much Democrats might prefer that they do so.